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Joint Coin Probability Problem

  1. May 30, 2010 #1
    Suppose that the probability that a head appears when a coin is tossed is p and the probability that a tail occurs is q=1-p. Person A tosses the coin until the first head appears and stops. Person B does likewise. The results obtained by persons A and B are assumed to be independent. What is the probability that A and B stop on exactly the same number toss?

    I am not quite sure how to solve this. I'm assuming that the tosses are distributed as a negative binomial and I'm lost as to how to put everything together.Please help !

  2. jcsd
  3. May 30, 2010 #2


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    Hi FaradayLaws! :smile:
    It's the sum from n = 1 to ∞ of the probability that A and B both stop on the nth toss. :wink:
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