Mathematics for Computer Science

16.6. References 691

3. p^1 n

4.

q

2

n

Exam Problems

Problem 16.2. (a)What’s the probability that 0 doesn’t appear amongkdigits
chosen independently and uniformly at random?

(b)A box contains 90 good and 10 defective screws. What’s the probability that
if we pick 10 screws from the box, none will be defective?

(c)First one digit is chosen uniformly at random fromf1;2;3;4;5gand is re-
moved from the set; then a second digit is chosen uniformly at random from the
remaining digits. What is the probability that an odd digit is picked the second
time?

(d)Suppose that yourandomlypermute the digits1;2;


;n, that is, you select
a permutation uniformly at random. What is the probability the digitkends up in
theith position after the permutation?

(e)A fair coin is flippedntimes. What’s the probability that all the heads occur
at the end of the sequence? (If no heads occur, then “all the heads are at the end of
the sequence” is vacuously true.)

Class Problems

Problem 16.3.
The New York Yankees and the Boston Red Sox are playing a two-out-of-three
series. In other words, they play until one team has won two games. Then that
team is declared the overall winner and the series ends. Assume that the Red Sox
win each game with probability3=5, regardless of the outcomes of previous games.
Answer the questions below using the four step method. You can use the same
tree diagram for all three problems.

(a)What is the probability that a total of 3 games are played?

(b)What is the probability that the winner of the series loses the first game?

(c)What is the probability that thecorrectteam wins the series?

Problem 16.4.
To determine which of two people gets a prize, a coin is flipped twice. If the flips