JEE Main & Advanced JEE Main Paper (Held On 15 April 2018) Slot-II

  • question_answer
    A player\[X\] has a biased coin whose probability of showing heads is p and a player Y has a fair coin. They start playing a game with their own coins and play alternately. The player who throws a head first is a winner. If X starts the game, and the probability of winning the game by both the players is equal, then the value of ?p? is                       [JEE Online 15-04-2018 (II)]

    A)         \[\frac{1}{3}\]                                   

    B) \[\frac{1}{5}\]

    C) \[\frac{1}{4}\]                   

    D)          \[\frac{2}{5}\]

    Correct Answer: A

    Solution :

    X wins when the outcome is one of the following set of outcomes: H, TTH, TTTTH,??.. Since subsequent tosses are independent, the probability that X wins is \[p+\frac{p}{4}+\frac{p}{16}+....=\frac{4p}{3}\] Similarly \[Y\] wins if the outcome is one of the following: \[TH,TTTH,TTTTTH,....\] So, the probability that Y wins is\[\frac{1-p}{2}+\frac{1-p}{8}+\frac{1-p}{32}=\frac{2(1-p)}{3}\] Since X and Y win with equal probability, we have \[\frac{4p}{3}=\frac{2(1-p)}{3}\Rightarrow p=\frac{1}{3}\]       So, option A is the correct answer.


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