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Geometric distribution problem

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  • Geometric distribution problem

    This is from the Guo study book

    You throw a dice repeatedly until you get a 6. What's the prob that you need to throw more than 20 times to get a 6?

    -----

    p = 1/6

    the solution says (1-1/6)^(20)

    but why isn't is (1/6)(1-1/6)^20 since the probability function is p(1-p)^x?

  • #2
    Originally posted by ixi View Post
    This is from the Guo study book

    You throw a dice repeatedly until you get a 6. What's the prob that you need to throw more than 20 times to get a 6?

    -----

    p = 1/6

    the solution says (1-1/6)^(20)

    but why isn't is (1/6)(1-1/6)^20 since the probability function is p(1-p)^x?
    Prob(more than 20 times to get a 6) = P(Not getting a 6 20 times in a row) = 5/6 * 5/6 * ... * 5/6 ~ 20 times.

    The way you described it would be answering P(out of 21 trials, 20 times not getting a 6 AND then getting a 6 once) or something like that
    Last edited by NoMoreExams; May 18 2010, 10:09 PM.

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    • #3
      oh i see. thanks

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