Introduction 550.420 to Probability
Fall 2011
Homework 3
Due Thursday September 22
1. (2 points) Clint Westwood has collected six old guns. The probability of hitting the
bulls-eye on a target when these guns are properly aimed and fired is 0.6, 0.5, 0.7, 0.9,
0.7, and 0.8, respectively. What is the probability of hitting the bulls-eye on a target if a
gun is selected at random, properly aimed, and fired?
2. (2 points) Suppose that cards are drawn at random from a standard deck of 52 playing
cards, one by one and without replacement. What is the probability that no heart is drawn
before the ace of spades?
3. (2 points) From a standard deck of 52 playing cards, cards are drawn one by one, at
random and without replacement. What is the probability that the fourth is heart drawn on
the tenth draw?
4. (2 points) A box contains 18 tennis balls, of which 8 are new. Suppose that three balls
are selected randomly, played with, and after play they are returned to the box. If another
three balls are selected for play a second time, what is the probability that they are all
new?
5. (2 points) Mustang Sally has two stables on her ranch, one that houses 20 horses and
13 mules, the other with 25 horses and 8 mules. Without any pattern, animals
occasionally leave their stables and then return to their stables. Suppose that during a
period when all the animals are in their stables, a horse comes out of a stable and then
returns. What is the probability that the next animal coming out of the same stable will
also be a horse?
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