# CS 70 Discrete Mathematics and Probability Theory Fall 2013

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```CS 70
Discrete Mathematics and Probability Theory
Fall 2013
Vazirani
Counting + Probability Practice
1. Counting, counting and counting
The only way to learn counting is to practice, practice, practice, so here is your chance to do so. We
encourage you to leave your answer as an expression (rather than trying to evaluate it to get a specific
number).
(a) How many 10-bit strings are there that contain exactly 4 ones?
(b) How many different 13-card bridge hands are there? (A bridge hand is obtained by selecting 13
cards from a standard 52-card deck. The order of the cards in a bridge hand is irrelevant.)
(c) How many different 13-card bridge hands are there that contain no aces?
(d) How many different 13-card bridge hands are there that contain all four aces?
(e) How many different 13-card bridge hands are there that contain exactly 6 spades?
(f) How many 99-bit strings are there that contain more ones than zeros?
(g) If we have a standard 52-card deck, how many ways are there to order these 52 cards?
(h) Two identical decks of 52 cards are mixed together, yielding a stack of 104 cards. How many
different ways are there to order this stack of 104 cards?
(i) How many different anagrams of FLORIDA are there? (An anagram of FLORIDA is any reordering of the letters of FLORIDA, i.e., any string made up of the letters F, L, O, R, I, D, and
A, in any order. The anagram does not have to be an English word.)
(j) How many different anagrams of ALASKA are there?
(k) How many different anagrams of ALABAMA are there?
(l) How many anagrams does the word PAPASAN have where the S and N are not adjacent? For
example, count APPSANA but do not count APAANSP.
(m) We have 9 balls, numbered 1 through 9, and 27 bins. How many different ways are there to
distribute these 9 balls among the 27 bins?
(n) We throw 9 identical balls into 7 bins. How many different ways are there to distribute these 9
balls among the 7 bins such that no bin is empty?
(o) How many different ways are there to throw 9 identical balls into 27 bins?
(p) How many ways are there to place 50 unlabeled balls in 9 labeled bins where each bin contains
at least as many balls as its bin number. (That is, bin 1 contains at least 1 ball, bin 2 contains at
least 2, and so on.)
(q) There are exactly 20 students currently enrolled in a class. How many different ways are there
to pair up the 20 students, so that each student is paired with one other student?
2. More Counting
Let A, B be finite sets, with |A| = m and |B| = n.
CS 70, Fall 2013, Counting + Probability Practice
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(a) How many subsets does A have? (Recall that the empty set and A are both subsets of A.)
(b) How many distinct functions f : A → B are there from A to B?
(c) Suppose m = n. How many distinct bijections are there from A to B?
3. And More Counting
How many non-negative integer solutions (x1 , . . . , x7 ) are there to the following equation?
x1 + x2 + x3 + x4 + x5 + x6 + x7 = 2003
x1 ≥ 0, . . . , x7 ≥ 0,
x1 , . . . , x7 ∈ Z
Order matters. For instance, (1, 2002, 0, 0, 0, 0, 0) counts as a different solution than (2002, 1, 0, 0, 0, 0, 0).
4. Sum of digits
Choose a number uniformly at random between 0 and 999,999, inclusive. What is the probability that
the digits sum to 19?
5. Algebraic vs. combinatorial proofs
Consider the following identity:
2n
n
=2
+ n2 .
2
2
(a) Prove the identity by algebraic manipulation (using the formula for the binomial coefficients).
(b) Prove the identity using a combinatorial argument. (Write both sides as the answer to a question
of the form “how many ways can you...?”)
6. Red cards
Consider a deck with just the four aces (red: hearts, diamonds; black: spades, clubs). Melissa shuffles
the deck and draws the top two cards.
Given that Melissa has the ace of hearts, what is the probability that Melissa has both red cards?
Given that Melissa has at least one red card, what is the probability that she has both red cards?
7. Sample Space and Events
Consider the sample space Ω of all outcomes from flipping a coin 4 times.
(a) List all the outcomes in Ω. How many are there?
(b) Let A be the event that the first flip is a Heads. List all the outcomes in A. How many are there?
(c) Let B be the event that the third flip is a Heads. List all the outcomes in B. How many are there?
(d) Let C be the event that the first flip and the third flip are both Heads. List all the outcomes in C.
How many are there?
(e) Let D be the event that the first flip or the third flip is a Heads. List all the outcomes in D. How
many are there?
(f) Are the events A and B disjoint? Express the event C in terms of A and B. Express the event D
in terms of A and B.
(g) Suppose now the coin is flipped n ≥ 3 times instead of 4 flips. Compute |Ω|, |A|, |B|, |C|, |D|.
CS 70, Fall 2013, Counting + Probability Practice
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8. Probability Models
Suppose you have two coins, one is biased with a probability of p coming up Heads, and one is biased
with a probability of q coming up Heads. Answer the questions below, but you don’t need to provide
justifications.
(a) Suppose p = 1 and q = 0.
i. You pick one of the two coins randomly and flip it. You repeat this process n times, each
time randomly picking one of the two coins and then flipping it. Consider the sample space
Ω of all possible length n sequences of Heads and Tails so generated. Give a reasonable
probability assignment (i.e. assign probabilities to all the outcomes) to model the situation.
ii. Now you pick one of the two coins randomly, but flip the same coin n times. Identify
the sample space for this experiment together with a reasonable probability assignment to
model the situation. Is your answer the same as in the previous part?
(b) Repeat the above two questions for arbitrary values of p and q. Express your answers in terms
of p and q.
9. Independence
We flip two unbiased coins: a nickel and a dime, and consider the following events:
(a) The nickel comes up heads.
(b) The dime comes up heads.
(c) The nickel and dime both come up heads.
(d) Exactly one of the nickel and dime comes up heads.
(e) The nickel and dime both come up the same way.
State without proof whether each of the following pairs of events are independent:
• (a) and (b):
• (a) and (c):
• (a) and (d):
• (a) and (e):
• (c) and (b):
• (d) and (b):
• (e) and (b):
• (c) and (d):
• (c) and (e):
• (d) and (e):
State without proof whether each of the following triples of events are independent:
• (a), (b), (c):
• (a), (b), (d):
• (a), (b), (e):
CS 70, Fall 2013, Counting + Probability Practice
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• (b), (d), (e):
• (a), (c), (e):
10. Correlation
It was suggested in class that, when Pr[A|B] > Pr[A], then A and B may be viewed intuitively as
being positively correlated. One might wonder whether “being positively correlated” is a symmetric
relation. Prove or disprove: If Pr[A|B] > Pr[A] holds, then Pr[B|A] > Pr[B] must necessarily hold, too.
(You may assume that both Pr[A|B] and Pr[B|A] are well-defined, i.e., neither Pr[A] nor Pr[B] are zero.)
11. Monty Hall Again
In the three-door Monty Hall problem, there are two stages to the decision, the initial pick followed by
the decision to stick with it or switch to the only other remaining alternative after the host has shown
an incorrect door. An extension of the basic problem to multiple stages goes as follow.
Suppose there are four doors, one of which is a winner. The host says: "You point to one of the doors,
and then I will open one of the other non-winners. Then you decide whether to stick with your original
pick or switch to one of the remaining doors. Then I will open another (other than the current pick)
non-winner. You will then make your final decision by sticking with the door picked on the previous
decision or by switching to the only other remaining door.
(a) How many possible strategies are there?
(b) For each of the possible strategies, calculate the probability of winning. What is the best strategy?
12. Smokers
A health study tracked a group of people for five years. At the beginning of the study, 20% were
classified as heavy smokers, 30% as light smokers, and 50% as nonsmokers. Results of the study
showed that light smokers were twice as likely as nonsmokers to die during the five-year study, but
only half as likely as heavy smokers.
Suppose we select, uniformly at random, a participant from this study, and it turns out that this participant died at some point during the five-year period. Calculate the probability that this participant
was classified as a heavy smoker at the beginning of the study. Show your calculation clearly.
13. The myth of fingerprints
A crime has been committed. The police discover that the criminal has left DNA behind, and they
compare the DNA fingerprint against a police database containing DNA fingerprints for 20 million
people. Assume that the probability that two DNA fingerprints (falsely) match by chance is 1 in 10
million. Assume that, if the crime was committed by someone whose DNA fingerprint is on file in
the police database, then it’s certain that this will turn up as a match when the police compare the
crime-scene evidence to their database; the only question is whether there will be any false matches.
Let D denote the event that the criminal’s DNA is in the database; ¬D denotes the event that the
criminal’s DNA is not in the database. Assume that it is well-documented that half of all such crimes
are committed by criminals in the database, i.e., assume that Pr[D] = Pr[¬D] = 1/2. Let the random
variable X denote the number of matches that are found when the police run the crime-scene sample
against the DNA database.
(a) Calculate Pr[X = 1|D].
CS 70, Fall 2013, Counting + Probability Practice
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(b) Calculate Pr[X = 1|¬D].
(c) Calculate Pr[¬D|X = 1]. Evaluate the expression you get and compute this probability to at least
two digits of precision.
As it happens, the police find exactly one match, and promptly prosecute the corresponding individual.
You are appointed a member of the jury, and the DNA match is the only evidence that the police
present. During the trial, an expert witness testifies that the probability that two DNA fingerprints
(falsely) match by chance is 1 in 10 million. In his summary statement, the prosecutor tells the jury
that this means that the probability that the defendant is innocent is 1 in 10 million.
(d) What is wrong with the prosecutor’s reasoning in the summary statement?
(e) Do you think the defendant should be convicted? Why or why not?
14. Poisoned pancakes
You have been hired as an actuary by IHOP corporate headquarters, and have been handed a report
from Corporate Intelligence that indicates that a covert team of ninjas hired by Denny’s will sneak
into some IHOP, and will have time to poison five of the pancakes being prepared (they can’t stay any
longer to avoid being discovered by Pancake Security). Given that an IHOP kitchen has 50 pancakes
being prepared, and there are ten patrons, each ordering five pancakes (which are chosen uniformly at
random from the pancakes in the kitchen), calculate the probabilities that the first patron:
(a) will not receive any poisoned pancakes;
(b) will receive exactly one poisoned pancake;
(c) will receive at least one poisoned pancake;
(d) will receive at least one poisoned pancake given that the second patron received at least one
poisoned pancake;
(e) Calculate the probability that any of the first three receive at least one poisoned pancake.
15. Colorful coins
We are given three coins. The first coin is a fair coin painted blue on the heads side and white on the
tails side. The other two coins are biased so that the probability of heads is p. They are painted blue
on the tails side and red on the heads side. One coin is randomly chosen and flipped twice.
(a) Describe the outcomes in the sample space, and give their probabilities. [N OTE: You may want
to draw a tree to illustrate the sample space.]
(b) Now suppose two coins are chosen randomly with replacement and each flipped once. Describe
the outcomes in the sample space in this new experiment, and give their probabilities. Are they
the same as in part (a)? [N OTE: You may want to draw a tree to illustrate the sample space.]
(c) Now suppose two coins are chosen randomly without replacement and each flipped once. Describe the outcomes in the sample space in this new experiment, and give their probabilities. Are
they the same as in parts (a) or (b)? [N OTE: You may want to draw a tree to illustrate the sample
space.]
(d) Suppose the probability that the two sides that land face up are the same color is
experiment in part (c). What does this tell you about the possible values of p?
CS 70, Fall 2013, Counting + Probability Practice
29
96
in the
5
(e) Let A be the event that you get a head on the first flip and B is the event that you get a head on
the second flip. In each of the experiments in (a), (b) and (c), determine whether A and B are
independent events.
16. A paradox in conditional probability?
Here is some on-time arrival data for two airlines, A and B, into the airports of Los Angeles and
Chicago. (Predictably, both airlines perform better in LA, which is subject to less flight congestion
and less bad weather.)
Los Angeles
Chicago
Airline A
# flights # on time
600
534
250
176
Airline B
#flights # on time
200
188
900
685
(a) Which of the two airlines has a better chance of arriving on time into Los Angeles? What about
Chicago?
(b) Which of the two airlines has a better chance of arriving on time overall?
(c) Do the results of parts (a) and (b) surprise you? Explain the apparent paradox, and interpret it in
terms of conditional probabilities.
17. A flippant choice
We have noted that if a fair coin is flipped three times, there are eight equally probable outcomes:
HHH, HHT, HTH, HTT, THH, THT, TTH, and TTT. Two CS 70 students play a game based on coin
flipping. Player A selects one of the triplets just listed; player B selects a different one. The coin
is then repeatedly flipped until one of the chosen triplets appears as a run and wins the game. For
example, if player A chooses HHT and player B chooses THT and the flips are THHHT, player A
wins.
Fill in the table below to show player B’s best choice of triplet for each possible choice that player A
makes, and the probability of player B winning with a best choice. Then explain why the odds for one
player winning are so lopsided.
Player A’s choice
HHH
HHT
HTH
HTT
THH
THT
TTH
TTT
Player B’s best choice
Player B’s probability of winning
18. Stakes well done
Two players, Alice and Bob, each stake 32 pistoles on a three-point, winner-take-all game of chance.
The game is played in rounds; at each round, one of the two players gains a point and the other gains
none. Normally the first player to reach 3 points would win the 64 pistoles. However, it starts to rain
during the game, and play is suspended at a point where Alice has 2 points and Bob has 1 point. Alice
and Bob have to figure out how to split the money.
CS 70, Fall 2013, Counting + Probability Practice
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You should assume that Alice and Bob are evenly matched, so that in each round Alice and Bob each
have a 50% chance of winning the round. Assume also that Alice’s share should be proportional to
the conditional expected valueof her winnings (specifically, her winnings if the game were continued
to the end from this point). The same goes for Bob.
Calculate a fair way to distribute the 64 pistoles using this notion of fairness. How many pistoles does
Alice receive? Bob?
CS 70, Fall 2013, Counting + Probability Practice
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