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Math Help - formulating this problem..

  1. #1
    MHF Contributor
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    formulating this problem..

    a man forgot the last 2 digits of a phone number.
    he only remembers that they are different
    and they are odd
    what is the chance that he will pick the correct combination of numbers
    of the original phone number

    \frac{\binom{5}{1}\cdot \binom{4}{1}}{\binom{10}{2}}=\frac{2}{9}

    is it a correct expression?
    Last edited by transgalactic; October 17th 2009 at 01:20 AM.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by transgalactic View Post
    a man forgot the last 2 digits of a phone number.
    he only remembers that they are different
    and they are odd
    what is the chance that he will pick the correct combination of numbers
    of the original phone number

    \frac{\binom{5}{1}\cdot \binom{4}{1}}{\binom{10}{2}}=\frac{2}{9}

    is it a correct expression?
    If they are both odd, there are 20 numbers to choose from, only one of which is correct, so the probability is ..

    CB
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  3. #3
    MHF Contributor
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    5 posibilities for first
    4 forthe second

    why 20 numbers to choose from
    the number are 0..9

    The digits are odd so we have {1, 3, 5, 7, 9} to choose them from. The first can be any of these five and the second any of the remaining 4 (since the digits are different) - so total is 5x4=20

    CB
    Last edited by CaptainBlack; October 17th 2009 at 02:16 AM.
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