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Unformatted text preview: CS237. Practice Problems Set 4: Basic Probability. Graded Problems due Thurs Feb 17 11:59PM. February 20, 2011 Reading. Schaums Chapter 4. L&L Chapter 1920. Optional Extra Practice. Any of the problems in Schaumss Chapter 4. MitzenmacherUpfal problems 1.11.16. 1 Practice problems, ungraded Hint. Use tree diagrams where ever possible! Exercise 1. There are three caves in Boston. A wolf is looking for a home; he moves into cave 1 with probability 5 9 , cave 2 with probability 1 9 and cave 3 with probability 1 9 , and with the remaining probability he moves to San Diego. A goat moves into one of the unoccupied caves, choosing his cave with equal probability. These caves are in Boston, so it snows with probability 9 10 . The goat always leaves tracks in the snow, but the wolf does not (hes clever and has learned how to cover his tracks). What is the probability that the wolf lives in cave 2 given that the there are no tracks in front of cave 1? Solution: Let W 1 , W 2 , W 3 and W S be the event that the wolf moves into caves 1, 2, 3 and San Diego respectively. Similarly, let G 1 , . . . , G 3 be the events that the goat moves into the caves 1 , 2 and 3 respectively. Let S be the event of snow, and S be the event of no snow. If there are no tracks in cave 1 (call this event A ), either the goat is not in cave 1, or it didnt snow; so we can make a tree diagram to solve this (first step  wolf choose cave, second step  goat chooses cave, third step  it snows or not). Draw the tree, youll find the following: Pr[ A ] = Pr[ G 1 S ] = Pr[ W 1 ] + Pr[ W 2 G 3 ] + Pr[ W 2 G 1 S ] + Pr[ W 3 G 2 ] + Pr[ W 3 G 1 S ] + (Pr[ W 4 ] Pr[ W 4 G 1 S ]) = 5 / 9 + (1 / 9)(1 / 2) + (1 / 9)(1 / 2)(1 / 10) + (1 / 9)(1 / 2) + (1 / 9)(1 / 2)(1 / 10) + (2 / 9 (2 / 9)(1 / 3)(9 / 10)) Now, we want the probability that the wolf is in cave 2 given no tracks in front of cave 1. Let event B be the event that the wolf is in cave 2. We want Pr[ B  A ] = Pr[ A B ] Pr[ A ] We already worked out Pr[ A ] . Referring again to our tree, we see that event A B contains only the outcomes { W 2 G 1 S,W 2 G 3 S,W 2 G 3 S } , so it occurs with probability Pr[ A B ] = (1 / 9)(1 / 2)(1 / 10) + (1 / 9)(1 / 2)(9 / 10) + (1 / 9)(1 / 2)(1 / 10) We get the answer after plugging the numbers into our calculators. Exercise 2. Dirty Harry puts 2 bullets in a 6cell cylinder of his revolver. He gives a cylinder a random spin and says Feeling lucky? He pulls the trigger. 1. Whats the probability that his victim gets shot? 1 2. Suppose his victim didnt get shot, but now Harry pulls the trigger again (without giving the gun another random spin). Whats the probability that his victim gets shot the second time Harry pulls the trigger?...
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This note was uploaded on 04/14/2011 for the course CS 237 taught by Professor Goldberg during the Spring '11 term at BU.
 Spring '11
 Goldberg

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