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Unformatted text preview: THE UNIVERSITY OF HONG KONG DEPARTMENT OF MATHEMATICS MATH2603 Probability Theory Solution to Assignment 3 Problem 26 You have two opponents with whom you alternate play. When ever you play A , you win with probability p A ; whenever you play B , you win with probability p B , where p B > p A . If your objective is to minimize the number of games you need to win two in a row, should you start with A or with B ? Solution: Let E [ N i ] denote the mean number of games needed if you initially play i ( i = A,B . Suppose we start with A , conditioning on the first two outcomes, we have E [ N A ] = 2 p A p B + p A (1 p B )( E [ N A ] + 2) + (1 p A )( E [ N B ] + 1) Similarly, E [ N B ] = 2 p B p A + p B (1 p A )( E [ N B ] + 2) + (1 p B )( E [ N A ] + 1) Subtracting the second equation from the first equation we have E [ N A ] E [ N B ] = p A p B + (1 p A )(1 p B )( E [ N B ] E [ N A ]) so E [ N A ] E [ N B ] = p A p B 1 + (1 p A )(1 p B ) < i.e. E [ N A ] < E [ N B ]. So we should start with A . Problem 69 In the match problem, say that ( i,j ) ,i < j , is a pair if i chooses j ’s hat and j chooses i ’s hat. a. Find the expected number of pairs. b. Let Q n denote the probability that there are no pairs, and derive a recursive formula for Q n in terms of Q j , j < n . c. Use the recursion of part (b) to find Q 8 ....
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This note was uploaded on 01/02/2012 for the course MATH 2603 taught by Professor Han during the Spring '10 term at HKU.
 Spring '10
 Han
 Probability

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