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Math Help - Prob of a letter bag from a set of letters

  1. #1
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    Prob of a letter bag from a set of letters

    Say I have a set of letters totaling n (say 80 for ex)
    (some letters may occur more than once in this set)

    I randomly choose r letters from n (say r= 4)
    What is the probability of seeing the letter bag
    'aabcd' (it's a bag so order doesn't matter, 'adcab' is also ok)
    Note that there are 2 a's in this bag.

    The general form of the bag is:
    k_a of letter a
    k_b of letter b
    ..
    all total to k

    Assuming that there are n_a of letters a
    n_b of letters b
    n_c of letters c
    and so on .. all total to n
    from the set of n letters

    My naive answer is
    [n_aCk_a * n_bCk_b * ... ] / nCr

    And I know this is not correct
    Can anybody help me get the correct answer for this?

    Thanks
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  2. #2
    MHF Contributor
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    Re: Prob of a letter bag from a set of letters

    Hey lettertha.

    Please look at your other thread for a response.
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