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Homework Help: Pretty Simple Probability Question

  1. Mar 26, 2009 #1
    The question is:

    A couple is trying to get a child. Their probability of conception in any given monthly cycle
    is 1/8
    (a) What is the mean number of cycles until conception occurs (the count includes the cycle
    in which conception occurs)? Neglect menopause and limited lifetime of the couple.
    (b) What is the smallest n such that the probability that it takes them more than n cycles
    is less than 1/2 ?

    I attempted to use a binomial distribution but realized that took me no where...I know it should be somewhere within 3-5 months but I can't prove it.

    I know this is simple, and that I'm likely over thinking.

    Thanks in advance.
  2. jcsd
  3. Mar 26, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    It sounds like a binomial distribution to me.

    Anyway, can you calculate what the probability P(n) is that conception takes place in the nth month? Because then you can simply calculate the expectation value of n through
    [tex]E = \sum_{n = 0}^\infty n P(n)[/tex]
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