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Math Help - Distributing items of different varieties

  1. #1
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    Distributing items of different varieties

    Find the number of ways to fill a box with a dozen doughnuts chosen from five
    varieties, with the requirement that at least one doughnut of each variety is
    picked.


    Here is how I think this is done, is this correct?
    1. Put one donut of each type in the box. 7 Donuts now left.
    2. These donuts can be divided using the | and * method. So *******|||| stands for 7 donuts and 4 separators for the varieties, 11 items in total. The dividers can be placed in 11 Choose 4 = 330 ways.

    Is this correct?
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  2. #2
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    Re: Distributing items of different varieties

    Quote Originally Posted by nukenuts View Post
    Find the number of ways to fill a box with a dozen doughnuts chosen from five varieties, with the requirement that at least one doughnut of each variety is picked.
    The best way to do these is to learn the multi-selection formula.
    The number of ways to put N identical items into K different cells is \binom{N+K-1}{N}.
    That is proven using the "stars & bars" as you suggested.

    In your problem the identical items are the choices and the different cells are the varieties of doughnuts. But we have the requirement that no cell be empty. So put one of each variety in the box. Now count the number of ways to make an addition seven choices: N=7~\&~K=5.
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  3. #3
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    Re: Distributing items of different varieties

    Cheers mate, good to know that there is a standard formula for this process.
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