Mathematical Models and Word Problems

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CHAPTER 3
Mathematical Models
and Word Problems
A great discovery solves a great problem but there is a grain of
discovery in the solution of any problem. Your problem may be
modest; but if it challenges your curiosity and brings into play your
inventive faculties, and if you solve it by your own means, you may
experience the tension and enjoy the triumph of discovery.
—George Polya
3.1
3.2
3.3
3.4
3.5
Coin Problems
Investment Problems
Distance (Uniform Motion) Problems
Mixture Problems
Critical Thinking Problems
Scientists and mathematicians work together to learn about, and hopefully
to solve, serious problems facing all of us. One of these problems is global
warming. The Earth’s surface is about one degree Fahrenheit warmer than
it was just 100 years ago. The presence in our atmosphere of increasing amounts
of so-called greenhouse gases like methane and carbon dioxide is largely
responsible.
If human activities like burning fossil fuels add methane to our air, how
do scientists determine what proportion of the atmosphere is methane? One
figure indicates that the concentration of methane has increased by 150% in
the last 250 years. What does this figure mean? Problems dealing with the
93
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CHAPTER 3
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Mathematical Models and Word Problems
concentrations of substances in combination are called mixture problems,
http://www.epa.gov
and they are one of the several types of word problems that this chapter
addresses. The chapter project will look at some more examples related to
global warming, and you can learn more at an informative government
website, www.epa.gov.
■ ■ ■
CHAPTER 3
■
Mathematical Models and Word Problems
When engineers want to test a new idea, they build a model upon which to experiment. A
properly constructed model is expected to behave in the same manner as the final product.
In the same way, a physicist may build a model that facilitates exploration of the behavior of
a natural phenomenon. Similarly, a biologist may build a model to explain the interaction
between body mechanisms.
Mathematicians also build models, but they do so on paper. A mathematical model
consists of mathematical expressions and equations that are an abstract representation of
the problem. The steps in the process consist of
(a) determining the variables
(b) creating the model
(c) using the model to find a solution or solutions
(d) verifying that the solution satisfies the original problem.
The steps in the process of mathematical modeling should sound familiar. They are precisely the steps we used in Chapter 2 when we introduced you to the intriguing world of
word problems. You have seen that the challenge in solving word problems lies in translating from words into mathematical equivalents, that is, in building the mathematical model.
We are now going to explore a variety of word problems in which we will show you
how to build a model in an organized manner that will lead to the appropriate algebraic
expressions and equations. In short, we are going to demonstrate a method for solving word problems that is virtually foolproof.
We pointed out in Section 2.2 that sometimes you must use a formula in order to solve a
problem. This need not frighten you. For example, you already know one of the formulas
we will use:
distance 5 rate × time
Often, these formulas express a relationship that you use all of the time but have never written down. For example, if you have a pocketful of change and want to know how wealthy
you are, you would determine the number of coins of each type (the technical term is denomination) and multiply each by the number of cents in that type. The formula
value in cents 5 number of coins × number of cents in each coin
explicitly states the relationship that you intuitively used.
Having successfully translated from words into algebra, you must now solve the equation that you have formulated. That’s the easy part: straightforward algebraic steps will lead
you to a numerical solution. The final step: always check to insure that the answer “makes
sense” in the context of the problem.
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Mathematical Models and Word Problems
3.1
Coin Problems
In building a model for coin problems, you must distinguish between the number of coins
and the value of the coins. For example,
n nickels have a value of 5n cents
n dimes have a value of 10n cents
n quarters have a value of 25n cents
If you have 8 quarters, what is their value? You find the answer by using this
relationship.
For any denomination of coins,
number of coins × number of cents in each coin 5 value in cents
Since each quarter has a value of 25 cents, the total value of the quarters is
8 × 25 5 200 cents
Example 1 A Coin Problem
A purse contains $3.20 in quarters and dimes. If there are 3 more quarters than dimes, how
many coins of each type are there?
Solution
In our model, we may let the unknown represent the number of quarters or the number of
dimes. We make a choice. Let
n 5 number of quarters
then
n 2 3 5 numbers of dimes
since “there are 3 more quarters than dimes.”
We can begin to build our model by gathering the data in the form of a chart, using the
relationship
value in cents 5 number of coins × number of cents in each coin
to guide us.
Number of coins
Quarters
Dimes
Total
n
n23
×
Number of cents in each coin
25
10
5
Value in cents
25n
10(n 2 3)
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Mathematical Models and Word Problems
In our problem, we are told that
total value 5 (value of quarters) 1 (value of dimes)
Substituting from the chart and solving,
320 5 25n 1 10(n 2 3)
320 5 25n 1 10n 2 30
350 5 35n
10 5 n
Then
n 5 number of quarters 5 10
n 2 3 5 number of dimes 5 7
Now verify that the value is $3.20.
✔ Progress Check 1
(a) Solve Example 1, letting the unknown n represent the number of dimes.
(b) A class collected $3.90 in nickels and dimes. If there were 6 more nickels than
dimes, how many coins were there of each type?
Answers
(a) 10 quarters, 7 dimes
(b) 24 dimes, 30 nickels
Example 2 A Coin Problem
A jar contains 25 coins worth $3.05. If the jar contains only nickels and quarters, how
many coins are there of each type?
Solution
We’ll choose a variable to represent the number of nickels:
n 5 number of nickels
Can our model represent the number of quarters in terms of n? Since there is a total of
25 coins, we must have
25 2 n 5 number of quarters
The model can then be built in the form of a chart.
Number of coins
Nickels
Quarters
Total
n
25 2 n
×
Number of cents in each coin
5
25
5
Value in cents
5n
25(25 2 n)
305
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CHAPTER 3
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Mathematical Models and Word Problems
We know that
total value 5 (value of nickels) 1 (value of quarters)
305 5 5n 1 25(25 2 n)
305 5 5n 1 625 2 25n
2320 5 220n
n 5 16 5 number of nickels
25 2 n 5 9 5 number of quarters
Verify that the coins have a total value of $3.05.
✔ Progress Check 2
A pile of coins worth $10 consisting of quarters and half-dollars is lying on a desk. If
there are twice as many quarters as half-dollars, how many half-dollars are there?
Answer
10
Example 3 A Disguised Coin Problem
A man purchased 10-cent, 15-cent, and 20-cent stamps with a total value of $8.40. If the
number of 15-cent stamps is 8 more than the number of 10-cent stamps and there are 10
more of the 20-cent stamps than of the 15-cent stamps, how many of each did he
receive?
Solution
This problem points out two things: (a) it is possible to phrase coin problems in terms of
stamps or other objects, and (b) a “wordy” problem can be attacked by the same type of
analysis.
Number of stamps
10-cent
15-cent
20-cent
×
Denomination of each stamp
n28
n
n 1 10
5
Value in cents
10(n 2 8)
15n
20(n 1 10)
10
15
20
Total
840
We let n be the number of 15-cent stamps (since the 10-cent and 20-cent stamps are
specified in terms of the 15-cent stamps). Since
1
2 1
2 1
2
value of
value of
value of
total value 5 10-cent stamps 1 15-cent stamps 1 20-cent stamps
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Mathematical Models and Word Problems
99
we have
840 5 10(n 2 8) 1 15n 1 20(n 1 10)
840 5 10n 2 80 1 15n 1 20n 1 200
840 5 45n 1 120
720 5 45n
16 5 n
Thus,
n 5 number of 15-cent stamps 5 16
n 2 8 5 number of 10-cent stamps 5 8
n 1 10 5 number of 20-cent stamps 5 26
Verify that the total value is $8.40.
✔ Progress Check 3
The pretzel vendor finds that her coin-changer contains $8.75 in nickels, dimes,
and quarters. If there are twice as many dimes as nickels and 10 fewer quarters
than dimes, how many of each kind of coin are there?
Answer
15 nickels, 30 dimes, and 20 quarters
Exercise Set 3.1
1. A soda machine contains $3.00 in nickels and dimes. If
the number of dimes is 5 times more than twice the
number of nickels, how many coins of each type are
there?
2. A donation box has $8.50 in nickels, dimes, and quarters. If there are twice as many dimes as nickels, and 4
more quarters than dimes, how many coins of each type
are there?
3. A wallet has $460 in $5, $10, and $20 bills. The number
of $5 bills exceeds twice the number of $10 bills by 4,
while the number of $20 bills is 6 fewer than the number of $10 bills. How many bills of each type are there?
4. A traveler buys $990 in traveler’s checks, in $10, $20,
and $50 denominations. The number of $20 checks is
3 less than twice the number of $10 checks, while the
number of $50 checks is 5 less than the number of $10
checks. How many traveler’s checks were bought in
each denomination?
5. A movie theater charges $5 admission for an adult and
$3 for a child. If 700 tickets were sold and the total revenue received was $2900, how many tickets of each type
were sold?
6. At a gambling casino a red chip is worth $5, a green one
$2, and a blue one $1. A gambler buys $27 worth of
chips. The number of green chips is 2 more than 3 times
the number of red ones, while the number of blue chips
is 3 less than twice the number of red ones. How many
chips of each type did the gambler get?
7. A student buys 5-cent, 10-cent, and 15-cent stamps,
with a total value of $6.70. If the number of 5-cent
stamps is 2 more than the number of 10-cent stamps,
while the number of 15-cent stamps is 5 more than one
half the number of 10-cent stamps, how many stamps
of each denomination did the student obtain?
8. A railroad car, designed to carry containerized cargo,
handles crates that weigh 1, 21 , and 41 ton. On a certain
day, the railroad car carries 17 tons of cargo. If the number of 21 -ton containers is twice the number of 1-ton containers, while the number of 41 -ton containers is 8 more
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Mathematical Models and Word Problems
than 4 times the number of 1-ton containers, how many
containers of each type are in the car?
9. An amateur theater group is converting a large classroom into an auditorium for a forthcoming play. The
group will sell $3, $5, and $6 tickets. They want to
receive exactly $503 from the sale of the tickets. If the
number of $5 tickets to be sold is twice the number of $6
tickets, and the number of $3 tickets is 1 more than 3
times the number of $6 tickets, how many tickets of
each type are there?
10. An amusement park sells 25-cent, 50-cent, and $1 tickets and a teacher purchases $32.50 worth of tickets. A
student remarks that there are twice as many 50-cent
tickets as there are $1 tickets and that the number of
25-cent tickets is 30 more than the number of 50-cent
tickets. How many tickets of each type are there?
11. During its annual picnic, a company supplies lemonade
for all employees and their families. The picnic committee has purchased twice as many pint jugs as quart jugs
and 8 fewer gallon jugs than quart jugs. How many jugs
of each type are there if 22 gallons of lemonade were
purchased? (Hint: There are 2 pints to a quart and 4
quarts to a gallon.)
12. A gym offers a variety of weights for use by its members. If there are 6 more 50-pound weights than 100pound weights and three times as many 20-pound
weights as 50-pound weights, for a total of 3180
pounds, how many of each weight are there?
3.2 Investment Problems
The class of investment problems that we are going to solve involves simple interest. As an
example, assume that you invest $500 (called the principal) at an annual interest rate of 6%.
Then the interest I available at year’s end is
I 5 (0.06)(500) 5 30
In this example, you have earned $30 in interest. We can generalize and develop a formula
that will form the basis for our modeling of these investment problems.
simple annual interest 5 principal × annual rate
or
I5P?r
This formula will be used in all investment problems.
Example 1 Investing at Simple Interest
A part of $7000 is invested at 6% annual interest and the remainder at 8%. If the total amount
of annual interest is $460, how much was invested at each rate?
Solution
Let
n 5 amount invested at 6%
then
7000 2 n 5 amount invested at 8%
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Mathematical Models and Word Problems
since the total amount is $7000. The model can then be built in the form of a chart.
×
Principal
Rate
n
7000 2 n
6% portion
8% portion
5
Interest
0.06n
0.08(7000 2 n)
0.06
0.08
Total
460
Since the total interest is the sum of the interest from the two parts,
460 5 0.06n 1 0.08(7000 2 n)
460 5 0.06n 1 560 2 0.08n
0.02n 5 100
n 5 $5000 5 portion invested at 6%
7000 2 n 5 $2000 5 portion invested at 8%
✔ Progress Check 1
A club decides to invest a part of $4600 in stocks earning 4.5% annual dividends,
and the remainder in bonds paying 7.5%. How much must the club invest in each
to obtain a net return of 5.4%?
Answer
$3220 in stocks, $1380 in bonds
Example 2 Investing at Simple Interest
A part of $12,000 is invested at 5% annual interest, and the remainder at 9%. The annual
income on the 9% investment is $100 more than the annual income on the 5% investment. How much is invested at each rate?
Solution
Let
n 5 amount invested at 5%
then
12,000 2 n 5 amount invested at 9%
We can then model the information in the form of a chart.
Principal
5% investment
9% investment
n
12,000 2 n
×
Rate
0.05
0.09
5
Interest
0.05n
0.09(12,000 2 n)
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Mathematical Models and Word Problems
Since the interest on the 9% investment is $100 more than the interest on the 5%
investment,
0.09(12,000 2 n) 5 0.05n 1 100
1080 2 0.09n 5 0.05n 1 100
980 5 0.14n
n 5 7000
Thus, $7000 is invested at 5% and $5000 at 9%.
✔ Progress Check 2
$7500 is invested in two parts yielding 5% and 15% annual interest. If the interest
earned on the 15% investment is twice that earned on the 5% investment, how
much is invested in each?
Answer
$4500 at 5%, $3000 at 15%
Example 3 An Inventory Investment Problem
A shoe store owner had $6000 invested in inventory. The profit on women’s shoes was
35%, while the profit on men’s shoes was 25%. If the profit on the entire stock was 28%,
how much was invested in each type of shoe?
Solution
Let
n 5 amount invested in women’s shoes
then
6000 2 n 5 amount invested in men’s shoes
In chart form, the model now looks like this:
Principal
Women’s shoes
Men’s shoes
Total stock
×
Rate
5
Profit
n
6000 2 n
0.35
0.25
0.035n
0.25(6000 2 n)
6000
0.28
0.28(6000)
The profit on the entire stock was equal to the sum of the profits on each portion:
0.28(6000) 5 0.35n 1 0.25(6000 2 n)
1680 5 0.35n 1 1500 2 0.25n
180 5 0.1n
n 5 1800
The store owner had invested $1800 in women’s shoes and $4200 in men’s shoes.
CHAPTER 3
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Mathematical Models and Word Problems
103
✔ Progress Check 3
An automobile dealer has $55,000 invested in compacts and midsize cars. The
profit on sales of the compacts is 10%, and the profit on sales of midsize cars is 16%.
How much did the dealer invest in compact cars if the overall profit on the total
investment is 12%?
Answer
$36,666.67
Exercise Set 3.2
1. A part of $8000 was invested at 7% annual interest, and
the remainder at 8%. If the total annual interest is $590,
how much was invested at each rate?
2. A $20,000 scholarship endowment fund is to be invested in two ways: part in a stock paying 5.5% annual
interest in dividends and the remainder in a bond paying 7.5%. How much should be invested in each to
obtain a net yield of 6.8%?
3. To help pay for his child’s college education, a father
invests $10,000 in two separate investments: part in a
certificate of deposit paying 8.5% annual interest, the
rest in a mutual fund paying 7%. The annual income on
the certificate of deposit is $200 more than the annual
income on the mutual fund. How much is invested in
each type of investment?
4. A bicycle store selling 3-speed and 10-speed models
has $16,000 in inventory. The profit on a 3-speed is 11%,
while the profit on a 10-speed model is 22%. If the profit
on the entire stock is 19%, how much was invested in
each type of bicycle?
5. A film shop carrying black-and-white film and color
film has $4000 in inventory. The profit on black-andwhite film is 12%, and the profit on color film is 21%.
If the annual profit on color film is $150 less than the
annual profit on black-and-white film, how much was
invested in each type of film?
6. A widow invested one third of her assets in a certificate
of deposit paying 6% annual interest, one sixth of her
assets in a mutual fund paying 8%, and the remainder
in a stock paying 8.5%. If her total annual income from
these investments is $910, what was the total amount
invested by the widow?
7. A trust fund has invested $8000 at 6% annual interest.
How much additional money should be invested at
8.5% to obtain a return of 8% on the total amount
invested?
8. A businessman invested a total of $12,000 in two ventures. In one he made a profit of 8% and in the other he
lost 4%. If his net profit for the year was $120, how
much did he invest in each venture?
9. A retiree invested a certain amount of money at 6%
annual interest; a second amount, which is $300 more
than the first amount, at 8%; and a third amount, which
is 4 times as much as the first amount, at 10%. If the
total annual income from these investments is $1860,
how much was invested at each rate?
10. A finance company lends a certain amount of money to
Firm A at 7% annual interest; an amount $100 less than
that lent to Firm A is lent to Firm B at 8%; and an
amount $200 more than that lent to Firm A is lent to
Firm C at 8.5%. If the total annual income is $126.50,
how much was lent to each firm?
11. A prospective bridegroom wants to buy an engagement
ring. Two jewelry stores each show him a ring at a cost
of $2400. One jeweler requires a 20% down payment
with the balance to be paid at the end of one year at 11%
simple interest. The other jeweler requires a 25% down
payment with the balance to be paid at the end of one
year at 12% simple interest. What is the difference in
total cost?
12. Because payment is one month overdue, a customer
receives a department store bill for $332.92 that includes
a 1.5% interest charge for late payment. What was the
original amount of the bill?
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13. An art dealer is ready to sell a Goya drawing and a
Monet watercolor for which he paid a total of $45,000.
If the Goya appreciated 83% and the Monet appreciated
72%, how much profit will he realize on each if he is
offered $80,700 for both?
computed after 6 months; then this interest is added to
the principal so that it may earn interest for the next 6
months. (See below.)
14. A small firm borrows $1000 from a stockholder at a simple interest rate of 7.5%. The company secretary lends
the firm an additional sum at a simple interest rate of
8.25%. At the end of one year, the firm repays a total of
$3997.75. How much did the secretary lend the firm and
what is the simple interest rate on the total loan?
The total interest for the year is $7.50 plus $7.56, or
$15.06, rather than the $15.00 we would expect from
simple interest.
Repeat the procedure above for quarterly (four times
per year) and monthly compounding. Put your findings
in a table.
15. Use your graphing calculator to investigate what happens if you calculate interest more often than once per
year (this is called compound interest). Suppose $1000 is
invested at an interest rate of 1.5 % and the interest is
3.3 Distance (Uniform Motion) Problems
Here is the basic formula for solving distance problems:
distance 5 rate × time
or
d5r?t
For instance, an automobile traveling at an average speed of 50 miles per hour for 3 hours
will travel a distance of
d5 r?t
5 50 ? 3 5 150 miles
The relationships that permit you to write an equation are sometimes obscured by
the words. Here are some questions to ask as you set up a distance problem:
(a) Are there two distances that are equal? Will two objects have traveled the same
distance? Is the distance on a return trip the same as the distance going?
(b) Is the sum (or difference) of two distances equal to a constant? When two
objects are traveling toward each other, they meet when the sum of the distances traveled by each equals the original distance between them.
Example 1 A Distance Problem
Two trains leave New York for Chicago. The first train travels at an average speed of
60 miles per hour, while the second train, which departs an hour later, travels at an
CHAPTER 3
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Mathematical Models and Word Problems
average speed of 80 miles per hour. How long will it take the second train to overtake
the first train?
Solution
Since we are interested in the time the second train travels, we choose to let
t 5 number of hours second train travels
then
t 1 1 5 number of hours first train travels
since the first train departs one hour earlier. In chart form, the model now looks like this:
Rate
First train
Second train
×
Time
5
t11
t
60
80
Distance
60(t 1 1)
80t
At the moment the second train overtakes the first, they must both have traveled the
same distance.
60(t 1 1) 5 80t
60t 1 60 5 80t
60 5 20t
35 t
It will take the second train 3 hours to catch up with the first train.
✔ Progress Check 1
A light plane leaves the airport at 9 A.M. traveling at an average speed of 200 miles
per hour. At 11 A.M. a jet plane departs and follows the same route. If the jet travels
at an average speed of 600 miles per hour, at what time will the jet overtake the
light plane?
Answer
12 noon
Warning
The units of measurement of rate, time, and distance must be consistent. If a car
travels at an average speed of 40 miles per hour for 15 minutes, then the distance
covered is
d5r?t
1
d 5 40 ? 5 10 miles
4
1
since 15 minutes 5 4 hour.
105
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Example 2 A Distance Problem
A jogger running at the rate of 4 miles per hour takes 45 minutes more than a car traveling at 40 miles per hour to cover a certain course. How long does it take the jogger to
complete the course and what is the length of the course?
Solution
Notice that time is expressed in both minutes and hours. Let’s choose hours as the unit
of time and let
t 5 time for the jogger to complete the course
then
3
5 time for the car to complete the course
4
since the car takes 45 minutes (5 43 hour) less time. We can then model the information
in the form of a chart.
t2
Rate
×
Time
Jogger
4
t
Car
40
t2
5
Distance
4t
3
4
1
40 t 2
Since the jogger and car travel the same distance,
1
4t 5 40 t 2
2
3
5 40t 2 30
4
30 5 36t
5
5 t
6
The jogger takes 56 hour or 50 minutes. The distance traveled is
4t 5 4 ?
5
20
1
5
5 3 miles
6
6
3
✔ Progress Check 2
The winning horse finished the race in 3 minutes; a losing horse took 4 minutes.
If the average rate of the winning horse was 5 feet per second more than the
average rate of the slower horse, find the average rates of both horses.
Answer
Winner: 20 feet per second; loser: 15 feet per second
3
4
2
CHAPTER 3
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Mathematical Models and Word Problems
107
Example 3 A Distance Problem
At 2 P.M. a plane leaves Boston for San Francisco, traveling at an average speed of 500
miles per hour. Two hours later a plane departs from San Francisco to Boston traveling
at an average speed of 600 miles per hour. If the cities are 3200 miles apart, at what time
do the planes pass each other?
Solution
Let
t 5 the number of hours after 2 P.M. at which the planes meet
Let’s piece together the information that we have. The model can then be built in the
form of a chart.
×
Rate
From Boston
From San Francisco
Time
5
t
t22
500
600
Distance
500t
600(t 2 2)
At the moment that the planes pass each other, the sum of the distances traveled by both
planes must be 3200 miles.
San Francisco
———————————>
—————————— 3200 miles ——————————
<————————————————
Thus,
3200 5 500t 1 600(t 2 2)
3200 5 500t 1 600t 2 1200
4400 5 1100t
45 t
The planes meet 4 hours after the departure of the plane from Boston.
✔ Progress Check 3
Two cyclists start at the same time from the same place and travel in the same
direction. If one cyclist averages 16 miles per hour and the second averages 20
miles per hour, how long will it take for them to be 12 miles apart?
Answer
3 hours
Boston
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Exercise Set 3.3
1. Two trucks leave Philadelphia for Miami. The first truck
to leave travels at an average speed of 50 kilometers per
hour. The second truck, which leaves 2 hours later, travels at an average speed of 55 kilometers per hour. How
long will it take the second truck to overtake the first
truck?
2. Jackie either drives or bicycles from home to school.
Her average speed when driving is 36 miles per hour,
and her average speed when bicycling is 12 miles per
hour. If it takes her 21 hour less to drive to school than to
bicycle, how long does it take to drive to school, how
long does it take to bicycle to school, and how far is the
school from her home?
3. Professors Roberts and Jones, who live 676 miles apart,
are exchanging houses and jobs for four months. They
start out for their new locations at exactly the same
time, and they meet after 6.5 hours of driving. If their
average speeds differ by 4 miles per hour, what is each
professor’s average speed?
4. Steve leaves school by moped for spring vacation. Forty
minutes later his roommate, Frank, notices that Steve
forgot to take his camera, so Frank decides to try to
catch up with Steve by car. If Steve’s average speed is
25 miles per hour and Frank averages 45 miles per hour,
how long does it take Frank to overtake Steve?
5. A tour boat makes the round trip from the mainland to
a fishing village in 6 hours. If the average speed of the
boat going to the village is 15 miles per hour and the
3.4
average speed returning is 12 miles per hour, how far
from the mainland is the island?
6. Two cars start out from the same point at the same time
and travel in opposite directions. If their average speeds
are 36 and 44 miles per hour, respectively, after how
many hours will they be 360 miles apart?
7. An express train and a local train start out from the
same point at the same time and travel in opposite
directions. The express train travels twice as fast as the
local train. If after 4 hours they are 480 kilometers apart,
what is the average speed of each train?
8. Two planes start out from the same place at the same
time and travel in the same direction. One plane has an
average speed of 400 miles per hour and the other plane
has an average speed of 480 miles per hour. After how
many hours will they be 340 miles apart?
9. Two cyclists start out at the same time from points
that are 395 kilometers apart and travel toward each
other. The first cyclist travels at an average speed of
40 kilometers per hour, and the second travels at an
average speed of 50 kilometers per hour. After how
many hours will they be 35 kilometers apart?
10. It takes a student 8 hours to drive from her home back
to college, a distance of 580 kilometers. Before lunch her
average speed is 80 kilometers per hour and after lunch
it is 60 kilometers per hour. How many hours does she
travel at each rate?
Mixture Problems
One type of mixture problem involves mixing commodities, say, two or more types of nuts,
to obtain a mixture with a desired value. To form a suitable model, we will need to use a
number of “common sense” relationships. If the commodities are measured in pounds,
these are
number of pounds × price per pound 5 value of commodity
pounds in mixture 5 sum of pounds of each commodity
value of mixture 5 sum of values of individual commodities
CHAPTER 3
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Mathematical Models and Word Problems
Example 1 A Coffee Mixture
How many pounds of Brazilian coffee worth $5 per pound must be mixed with 20 pounds
of Colombian coffee worth $4 per pound to produce a mixture worth $4.20 per pound?
Solution
Let
n 5 number of pounds of Brazilian coffee
The model can then be built in the form of a chart.
Type of coffee
Brazilian
Colombian
Mixture
Number of pounds
×
Price per pound
5
Value in cents
n
20
500
400
500n
8000
n 1 20
420
420(n 1 20)
(Note that the weight of the mixture equals the sum of the weights of the Brazilian and
Colombian coffees going into the mixture.) Since the value of the mixture is the sum of the
values of the two types of coffee, we have
420(n 1 20) 5 500n 1 8000
420n 1 8400 5 500n 1 8000
400 5 80n
55 n
We must add 5 pounds of Brazilian coffee.
✔ Progress Check 1
How many pounds of macadamia nuts worth $4 per pound must be mixed with
4 pounds of cashews worth $2.50 per pound and 6 pounds of pecans worth $3 per
pound to produce a mixture that is worth $3.20 per pound?
Answer
5 pounds
Example 2 A Mixture of Chocolates
Caramels worth $1.75 per pound are to be mixed with cream chocolates worth $2 per
pound to make a 5-pound mixture that will be sold at $1.90 per pound. How many
pounds of each are needed?
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Mathematical Models and Word Problems
Solution
Let n 5 number of pounds of caramels. Displaying all of the information, we have
Type of candy
Caramels
Cream chocolates
Mixture
×
Number of pounds
5
Price per pound
Value in cents
n
52n
175
200
175n
200(5 2 n)
5
190
950
(Note that the number of pounds of cream chocolates is the weight of the mixture less
the weight of the caramels.) Since the value of the mixture is the sum of the values of
the two components, we have
950 5 175n 1 200(5 2 n)
950 5 175n 1 1000 2 200n
25n 5 50
n52
We must have 2 pounds of caramels and 3 pounds of cream chocolates.
✔ Progress Check 2
How many gallons of oil worth 55¢ per gallon and how many gallons of oil
worth 75¢ per gallon must be mixed to obtain 40 gallons of oil worth 60¢ per
gallon?
Answer
30 gallons of the 55-cent oil and 10 gallons of the 75-cent oil
Liquid Mixtures
A second type of mixture problem involves solutions containing different concentrations of materials. For instance, a 40-gallon drum of a solution that is 75% acid contains
(40)(0.75) 5 30 gallons of acid. If the solutions are measured in gallons, the relationship
we need is
number of
gallons of
solution
×
% of
component A
5
number of
gallons of
component A
The other relationships we need are really the same as in our first type of mixture
problem.
CHAPTER 3
number of gallons
in mixture
number of gallons
of component A
in mixture
■
Mathematical Models and Word Problems
5
sum of the number of gallons
in each solution
5
sum of the number of gallons
of component A in each
solution
111
Example 3 A Liquid Mixture
A 40% acid solution is to be mixed with a 75% acid solution to produce 140 gallons of a
solution that is 50% acid. How many gallons of each solution must be used?
Solution
For our model, we let
n 5 number of gallons of the 40% acid solution
Then
140 2 n 5 number of gallons of the 75% acid solution
since the number of gallons in the mixture is the sum of the number of gallons in each
contributing solution. We can then model the information in the form of a chart.
Number of gallons
40% solution
75% solution
Mixture
×
% acid
5
Number of gallons of acid
n
140 2 n
40
75
0.40n
0.75(140 2 n)
140
50
70
Since the number of gallons of acid in the mixture is the sum of the number of gallons
of acid in each solution (see Figure 1), we have
70 5 0.40n 1 0.75(140 2 n)
70 5 0.40n 1 105 2 0.75n
235 5 20.35n
n 5 100 gallons
140 2 n 5 40 gallons
Thus, we mix 100 gallons of the 40% solution with 40 gallons of the 75% solution to produce 140 gallons of the 50% solution.
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Mathematical Models and Word Problems
+
=
40%
acid
75%
acid
50%
acid
n gallons
140 – n gallons
140 gallons
Figure 1 Concentration of Acid
✔ Progress Check 3
How many gallons of milk that is 22% butterfat must be mixed with how many
gallons of cream that is 60% butterfat to produce 19 gallons of a mixture that is
40% butterfat?
Answer
10 gallons of milk and 9 gallons of cream
Example 4 A Mixture of Alloys
How many ounces of an alloy that is 30% tin must be mixed with 15 ounces of an alloy
that is 12% tin to produce an alloy that is 24% tin?
Solution
Let n 5 number of ounces of the 30% tin alloy. The alloy may be treated as a solution,
and we can display the information for our model.
Number of ounces
30% alloy
12% alloy
Mixture
×
% tin
5
Number of ounces of tin
n
15
30
12
0.30n
1.8
n 1 15
24
0.24(n 1 15)
(Note that the number of ounces in the mixture is the sum of the number of ounces in
the alloys going into the mixture.) Since the number of ounces of tin in the mixture is
the sum of the number of ounces of tin in each alloy, we have
CHAPTER 3
■
Mathematical Models and Word Problems
0.24(n 1 15) 5 0.30n 1 1.8
0.24n 1 3.6 5 0.30n 1 1.8
1.8 5 0.06n
n 5 30
Thus, we need to add 30 ounces of the 30% alloy to 15 ounces of the 12% alloy.
✔ Progress Check 4
How many pounds of a 25% copper alloy must be added to 50 pounds of a 55%
copper alloy to produce an alloy that is 45% copper?
Answer
We need to add 25 pounds of 25% copper alloy.
Example 5 Reducing a Mixture
A tank contains 40 gallons of water and 10 gallons of alcohol. How many gallons of
water must be removed if the remaining solution is to be 30% alcohol?
Solution
Let n 5 number of gallons of water to be removed. This problem is different since we
are removing water. We can model the information in the form of a chart.
Number of gallons
Original solution
Water removed
New solution
×
% alcohol
5
Gallons of alcohol
50
n
20
0
10
0
50 2 n
30
0.3(50 2 n)
(Note that the water removed has 0% alcohol!) The number of gallons of alcohol in the
new solution is the same amount as in the original solution, since only water has been
removed.
0.3(50 2 n) 5 10
15 2 0.3n 5 10
5 5 0.3n
2
n 5 16
3
Thus, we must remove 16 23 gallons of water.
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Mathematical Models and Word Problems
✔ Progress Check 5
A tank contains 90 quarts of an antifreeze solution that is 50% antifreeze.
How much water should be removed to raise the antifreeze level to 60% in the new
solution?
Answer
15 quarts of water should be removed.
Exercise Set 3.4
1. How many pounds of raisins worth $1.50 per pound
must be mixed with 10 pounds of peanuts worth $1.20
per pound to produce a raisin-peanut mixture worth
$1.40 per pound?
2. How many ounces of Ceylon tea worth $1.50 per ounce
and how many ounces of Formosa tea worth $2.00 per
ounce must be mixed to obtain a mixture of 8 ounces
that is worth $1.85 per ounce?
3. A copper alloy that is 40% copper is to be combined
with a copper alloy that is 80% copper to produce 120
kilograms of an alloy that is 70% copper. How many
kilograms of each alloy must be used?
4. How many liters of an ammonia solution that is 20%
ammonia must be mixed with 20 liters of an ammonia
solution that is 48% ammonia to produce a solution that
is 36% ammonia?
5. A vat contains 60 gallons of a 15% saline solution. How
many gallons of water must be evaporated so that the
resulting solution will be 20% saline?
6. How many grams of pure silver must be added to 30
grams of an alloy that is 50% silver to obtain an alloy
that is 60% silver?
7. How much water must be added to dilute 10 quarts of a
solution that is 18% iodine so that the resulting solution
will be 12% iodine?
8. A vat contains 27 gallons of water and 9 gallons of acetic
acid. How many gallons of water must be evaporated if
the remaining solution is to be 40% acetic acid?
9. How many pounds of a fertilizer worth $3 per pound
must be combined with 12 pounds of a weed killer
worth $6 per pound and 18 pounds of phosphate worth
$6 per pound to produce a mixture worth $4.80 per
pound?
10. A producer of packaged frozen vegetables wants to
market the product at $1.20 per kilogram. How many
kilograms of green beans worth $1 per kilogram must
be mixed with 100 kilograms of corn worth $1.30 per
kilogram and 90 kilograms of peas worth $1.40 per
kilogram to produce the required mixture?
CHAPTER 3
3.5
■
Mathematical Models and Word Problems
Critical Thinking Problems
In previous sections of this chapter, we discussed how to solve problems involving
coins, interest, mixtures, and distance. We will now use what we have learned to solve
additional critical thinking problems. It is best to utilize the following general steps in
order to solve problems more easily and consistently:
1. Understand the problem.
2. Translate the problem into an algebraic equation.
General
Steps for
Problem Solving
3. Solve the resulting equation.
4. Interpret the result.
Now let’s try some examples. The first example involves percentages.
You may be surprised at how often we use percentages in our everyday lives. For
example, if items are on sale, we get certain amounts of discount. It is easier to say that
we get a 15% discount rather than give the exact cost. You may also notice that realtors
usually speak in terms of percentages. For example, they say house prices increased by
10% last year or house prices are dropping by 5% this year.
It is important to know how to calculate the percent of increase or decrease.
Example 1 Calculating Home Prices
The average price of a home in Santa Maria was $200,000 in 1999. The average home
price increased to $450,000 in 2005. Find the percent increase.
Solution
Step 1. Understand the Problem. First, let’s read the problem. The average home price
has increased from $200,000 to $450,000 in Santa Maria. Now we need to find the percent
increase. Since we don’t know by what percent the average home price has increased,
we let x 5 the percent increase.
Step 2. Translate the Problem into an Equation. Let’s write the problem in words first
and then translate the words into an algebraic equation.
Increase 5 New average home price 2 Old average home price
Increase 5 $450,000 2 $200,000
Increase 5 $250,000
$250,000 is the amount of increase. To find the percent increase, we write the problem in
words first.
Increase 5 what percent increase
$250,000 5 x ? $200,000
of old average home price
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Mathematical Models and Word Problems
Step 3. Solve the Equation.
250,000 5 x ? 200,000
Since “of” represents “multiplication”
250,000
5x
200,000
Dividing both sides by 200,000
x 5 1.25
x 5 125%
Write as a percent
Step 4. Interpret the Results. Finally, check the proposed solution. The percent increase
in average home prices is 125%.
✔ Progress Check 1
If the price of a certain computer decreases from $2,000 to $1,200, find the percent
decrease.
Answer
40%
Example 2 Finding the Number of Tickets
For the musical called “The Company,” tickets cost $8 for children and $12 for adults.
Altogether 172 tickets were sold and $1,824 was collected. Find the number of child and
adult tickets sold.
Solution
After carefully reading the problem, we see that we need to find the number of child and
adult tickets sold. Since we don’t know theses two values, let’s write one of them as an
unknown.
Step 1. Understand the Problem.
Let x = the number of child tickets that were sold.
Then 172 2 x 5 the number of adult tickets that were sold.
Because
child tickets 1 adult tickets 5 172
x 1 adult tickets 5 172
adult tickets 5 172 2 x
Since each child ticket costs $8,
Revenue from child tickets 5 8 ? x
Since each adult ticket costs $12,
Revenue from adult tickets 5 12 ? (172 2 x)
CHAPTER 3
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Mathematical Models and Word Problems
Step 2. Translate the Problem into an Equation. Now, write the equation in words.
total revenue 5 revenue from child tickets 1 revenue from adult tickets
Then we can translate the words into the algebraic equation.
$1,824 5 8 ? x 1 12 ? (172 2 x)
Step 3. Solve the Equation.
Next, solve the equation for x.
1,824 5 8 ? x 1 12 ? (172 2 x)
1,824 5 8 ? x 1 2,064 2 12 ? x
By distributing 12 to (172 2 x)
1,824 5 24x 1 2,064
Combining like terms on the
right-hand side
1,824 2 2,064 5 24x
Subtracting 2,064 from both sides
2240 5 24x
2240
5x
24
Dividing both sides by 24
x 5 60
The number of adult tickets 5 172 2 x
5 172 2 60
5 112
Step 4. Interpret the Result. Let’s check the solution before we state the conclusion.
Check: By substituting x 5 60 into the equation $1,824 5 8 ? x 1 12 ? (172 2 x),
$1,824 5 8 ? (60) 1 12 ? (172 2 60)
$1,824 5 480 1 12 ? (112)
$1,824 5 480 1 1,344
$1,824 5 $1,824
Finally, we state the conclusion.
State: There were 60 child tickets and 112 adult tickets sold.
✔ Progress Check 2
a. The sum of two numbers is 60 and their difference is 24. Find the two
numbers.
b. Ms. Chung purchased 64 stamps, some 41-cent stamps and some 3-cent stamps.
Find the number of each type purchased if she spent $8.00 total.
Answers
a. 18 and 42
b. 16 41-cent stamps and 48 3-cent stamps
■ ■ ■
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Mathematical Models and Word Problems
Key Ideas for Review
Topic
Key Idea
Mathematical model
When you convert a word problem into mathematical expressions, equations, and
inequalities, you create a mathematical model for the problem.
Formulas associated with
word problems
coin problems
number of
coins
×
number of cents
in each coin
5
value in
cents
principal × rate 5 interest
investment problems
distance (uniform
motion) problems
rate × time 5 distance
mixture problems
number of pounds × price per pound 5 value
amount of
solution
×
% of
component A
5
amount of
component A
Review Exercises
Solutions to exercises whose numbers are in bold are in the Solutions section in the back of the book.
3.1
1. A church collection box contains $5.35 in dimes,
quarters, and half-dollars. If the number of dimes
is twice the number of quarters, and the number of
half-dollars is one less than three times the number
of quarters, how many coins of each denomination
are there?
2. A certain electronic device consists of 16-transistor,
48-transistor, and 64-transistor components. The
number of 48-transistor components is two less
than the number of 16-transistor components, and
the number of 64-transistor components is three
less than twice the number of 16-transistor components. If the device contains a total of 480 transistors, how many components of each type are
required?
3. A freighter carries a load of 2-ton, 5-ton, and 8-ton
slabs of steel. The number of 5-ton slabs is 20 fewer
than the number of 2-ton slabs and the number of
8-ton slabs is one more than twice the number of
2-ton slabs. If the load being carried weighs 575 tons,
how many slabs of each type are being carried?
4. Suppose that you receive a package with a total
postage of $5.60 that is made up of 20-cent, 40-cent,
and 1-dollar stamps. If the number of 40-cent
stamps is three more than the number of 20-cent
stamps, and the number of 1-dollar stamps is two
fewer than the number of 20-cent stamps, how
many stamps of each denomination are there on
the package?
3.2
5. Part of a lump-sum death payment of $40,000 was
invested at 10% annual interest, and the rest was
invested at 8% annual interest. If the total annual
interest is $3500, how much was invested at each
rate?
6. A record shop selling classical and popular music
has $12,000 worth of music in inventory. The profit
on classical music is 15%, while the profit on popular music is 20%. If the annual profit on classical
music is $1000 less than the annual profit on popular music, how much inventory does the shop carry
in each type of music? (Assume that the entire
inventory is sold.)
7. An investment club invested a total of $7000
in two real estate limited partnerships. In one partnership they make a profit of 10% for the year and
in the other they have a loss of 5% for the year. If
the net annual profit is $325, how much was
invested in each limited partnership?
CHAPTER 3
8. A finance company lent a certain amount of
money to the AB company at 8% annual interest.
An amount $500 more than that lent to the AB
company was lent to the CD company at 10%
annual interest, and an amount $400 less than the
amount lent to the AB company was lent to the
EF company at 12% annual interest. If the total
annual interest received by the finance company is
$1502, how much was lent to each borrower?
3.3
9. Two aircraft start from the same point at the same
time flying in opposite directions. The faster aircraft travels twice as fast as the other one. After
5 hours of travel they are 1500 miles apart. Find the
average speed of each aircraft.
10. Two joggers start to run toward each other at the
same time from points that are 12 miles apart, at
average speeds of 10 miles per hour and 9 miles
per hour, respectively. After how many hours will
they be 0.6 mile apart?
11. Two buses, traveling at average speeds of 50 and 55
miles per hour, respectively, leave Los Angeles for
Chicago at the same time. After how many hours
are they 60 miles apart?
12. Two airplanes leave at the same time from points
3150 miles apart, traveling toward each other, and
they pass each other after 3.5 hours of flying. If
their average speeds differ by 100 miles per hour,
what is the average speed of each airplane?
■
Mathematical Models and Word Problems
119
17. Atmospheric concentrations are usually measured
in parts per billion, by volume (ppbv). So, a concentration of carbon dioxide equal to 13 ppbv
means that, for every billion volume units (say,
milliliters) of the atmosphere, there are 13 units
of carbon dioxide.
Convert an atmospheric concentration of carbon
dioxide of 13 ppbv to a percent (that is, what percent of this atmosphere would be carbon dioxide?)
18. In 2002 the atmospheric concentration of the greenhouse gas methane was about 1751 parts per billion by volume (ppbv). This means that in a
one-billion liter volume of the atmosphere, we
should find about 1751 liters of methane gas.
Suppose we had a 100 L sample of gas with this
concentration of methane. How much methane, in
mL, would have to be added to increase the concentration to 150% of its current level? (Hint: We
want to increase the concentration to 1.5 × 1751,
or 2626.5 ppbv.)
19. Store the following values in your graphing calculator as A and B: A 5 0.1, B 5 0.3 (see the calculator
screen below). Then, enter the expression
112A 1 (172 2 112)B
172
Suppose the 112 in the above expression represents liters of CO2 , and the values of A and B represent 10% and 30%, respectively.
3.4 13. How many pounds of cashews worth $4.00 per
pound must be mixed with 6 pounds of walnuts
worth $2.00 per pound to yield a mixture worth
$2.50 per pound?
14. A vat contains 100 gallons of a 20% potassium solution. How many gallons of water must be evaporated to get a 25% solution?
a. What might the 172 represent? What about the
0.17?
15. How many pounds of ground beef that is 25% fat
must be blended with 10 pounds of ground veal
that is 10% fat to produce a mixture that is 15% fat?
b. Experiment with changing the values of A and
B. How can this expression help you solve certain mixture problems?
16. How many pounds of Colombian coffee worth
$4.00 per pound must be mixed with how many
pounds of Jamaican coffee worth $5.00 per pound
to produce 25 pounds of a mixture that will be sold
at $4.80 per pound?
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Mathematical Models and Word Problems
Progress Test 3A
1. Translate into algebra: “The number of chairs is 3 less
than 4 times the number of tables.”
2. Steve is presently 6 years younger than Lisa. If the sum
of their ages is 40, how old is each?
3. The width of a rectangle is 4 cm less than twice its
length. If the perimeter is 12 cm, find the dimensions of
the rectangle.
4. A donation box contains 30 coins consisting of nickels,
dimes, and quarters. The number of dimes is 4 more
than twice the number of quarters. If the total value of
the coins is $2.60, how many coins of each type are
there?
5. A fruit grower ships crates of oranges that weigh 30, 50,
and 60 pounds each. A certain shipment weighs 1140
pounds. If the number of 30-pound crates is 3 more than
one half the number of 50-pound crates, and the number of 60-pound crates is 1 less than twice the number of
50-pound crates, how many crates of each type are
there?
6. A college fund has invested $12,000 at 7% annual interest. How much additional money must be invested at
9% to obtain a return of 7.8% on the total amount
invested?
7. A businessperson invested a certain amount of money
at 6.5% annual interest; a second amount, which is $200
more than the first amount, at 7.5%; and a third amount,
which is $300 more than twice the first amount, at 9%.
If the total annual income from these investments is
$1962, how much was invested at each rate?
8. A moped and a car leave from the same point at the
same time and travel in opposite directions. The car
travels 3 times as fast as the moped. If after 5 hours they
are 300 miles apart, what is the average speed of each
vehicle?
9. A bush pilot in Australia picks up mail at a remote village and returns to home base in 4 hours. If the average
speed going is 150 miles per hour and the average
speed returning is 100 miles per hour, how far from the
home base is the village?
10. An alloy that is 60% silver is to be combined with
an alloy that is 80% silver to produce 120 ounces of
an alloy that is 75% silver. How many ounces of each
alloy must be used?
11. A beaker contains 150 cubic centimeters of a solution
that is 30% acid. How much water must be evaporated
so that the resulting solution will be 40% acid?
Progress Test 3B
1. Translate into algebra: “The number of Democrats is 4
more than one third the number of Republicans.”
2. Separate 48 into two parts so that the larger part plus 3
times the smaller is 80.
3. One side of a triangle is 2 cm shorter than the third side,
while the second side is 3 cm longer than one half the
third side. If the perimeter is 15 cm, find the length of
each side.
4. An envelope contains 20 discount coupons in $1, $5,
and $10 denominations. The number of $5 coupons is
twice the number of $10 coupons. If the total value of
the coupons is $54, how many coupons of each type are
there?
5. A cheese sampler with a total weight of 25 ounces of
cheese contains 1-ounce, 2-ounce, and 3-ounce samples.
If the number of 1-ounce samples is 3 more than the
number of 3-ounce samples, and the number of
2-ounce samples is 1 less than twice the number of
3-ounce samples, how many samples of each weight
are there?
6. Part of an $18,000 trust fund is to be invested in a stock
paying 6% in dividends, and the remainder in a bond
paying 7.2% annual interest. How much should be
invested in each to obtain a net yield of 7%?
7. A woman invested a certain amount of money at 8%
annual interest, and a second amount of money, $2000
greater than the first amount, at 6%. If the annual
incomes on the two investments are equal, how much
was invested at each rate?
8. Two trains start out at 10 A.M. from stations that are
1120 kilometers apart, and travel toward each other at
CHAPTER 3
average speeds of 80 and 60 kilometers per hour,
respectively. At what time will they pass each other?
9. Two charter buses leave New York for Los Angeles. The
first one travels at an average speed of 40 miles per
hour. The second one leaves 3 hours later and travels at
an average speed of 50 miles per hour. How long will it
take the second bus to overtake the first one?
■
Mathematical Models and Word Problems
121
10. How many pounds of lawn seed worth $4.00 per
pound must be mixed with 15 pounds of fertilizer
worth $3.00 per pound to produce a mixture worth
$3.20 per pound?
11. A vat contains 12 gallons of acid and 48 gallons of
water. How much acid must be added to make a solution that is 40% acid?
Chapter 3 Project: Greenhouse Gas Concentrations and Global Warming
The atmosphere of the Earth is a mixture of gases. The relative proportions of the
major components of the atmosphere (nitrogen and oxygen) don’t change very much
over time, but there are small amounts of other gases that can have a big effect on the
planet’s climate, and on its inhabitants (us!).
Greenhouse gases like carbon dioxide and methane trap heat that would otherwise be radiated out into space. A little of this is a good and necessary thing, but some
human activities (like burning of fossil fuels) release these gases and increase the concentration of greenhouse gases. This has the world’s scientists concerned that we may
have too much of a good thing!
In this chapter project, you will learn about how concentrations of these gases are
measured, and how to use the notion of percent to get an idea of what proportion of
our atmosphere is composed of greenhouse gases. You will try to determine how
much this proportion can change with the addition of greater quantities of these
atmospheric components. Start by reading more at www.epa.gov, and then do Exercises 17, 18, and 19 in Section 3.4.
Create a table showing how much the composition of the atmosphere changes
with the addition of different amounts of CO2 and methane. (You might want to use
scientific notation to express small percents more compactly.) Explain how you can
use your calculator to carry out these computations more quickly. (Hint: See Exercise
19 in Section 3.4.)
Using the information at the website listed above, or other sources, make a graph
of the changes in quantities of methane in our atmosphere over the past 200 years.

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