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Math Help - Putting items in bags

  1. #1
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    Putting items in bags

    Suppose there are 20 items and 12 bags. How many ways can the items be put into the bags if the items are distinct.



    Could someone offer me a little direction on this problem. Thanks
    Last edited by battery88; February 11th 2013 at 10:03 AM.
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    Re: Putting items in bags

    have a look at permutations, it is really straightforward, take a look and have an attempt I will show you the answer then
    Last edited by MathJack; February 11th 2013 at 10:32 AM.
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  3. #3
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    Re: Putting items in bags

    Quote Originally Posted by battery88 View Post
    Suppose there are 20 items and 12 bags. How many ways can the items be put into the bags if the items are distinct.
    I think the answer is C(20, 12), assuming the bags are also distinct.
    Could someone offer me a little direction on this.
    First, why do you assume that the bags are distinct? That is not part of the statement.

    If the items are distinct and the bags are distinct, then that is simply counting the number of functions from a set of twenty to a set of twelve:
    12^{20}.
    But that answer means some bags may be empty.

    If you require no empty bags, then that is simply counting the number of surjections from a set of twenty to a set of twelve:
    \sum\limits_{k = 0}^{12} {\left( { - 1} \right)^k \binom{ 12}{k} (12 - k)^{20} }.

    BUT, I don't think it means that the bags are distinct.
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    Re: Putting items in bags

    Okay, the 1st item can be placed 12 ways,
    the 2nd item can be placed 12 ways,
    the 3rd item can be placed 12 ways,
    and so forth, until we have 12^20 ways.
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    Re: Putting items in bags

    Quote Originally Posted by Plato View Post
    First, why do you assume that the bags are distinct? That is not part of the statement.

    If the items are distinct and the bags are distinct, then that is simply counting the number of functions from a set of twenty to a set of twelve:
    12^{20}.
    But that answer means some bags may be empty.

    If you require no empty bags, then that is simply counting the number of surjections from a set of twenty to a set of twelve:
    \sum\limits_{k = 0}^{12} {\left( { - 1} \right)^k \binom{ 12}{k} (12 - k)^{20} }.

    BUT, I don't think it means that the bags are distinct.
    I was thinking of something else: placing distinguished items in distinguished boxes.
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    Re: Putting items in bags

    So, if the items were identical would the number of ways to place the items in the bags = 12^1 = 12?
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    Re: Putting items in bags

    I'm pretty sure my previous post was incorrect, so how would I solve this problem if the items are identical. In other words:

    Suppose there are 20 items and 12 bags. How many ways can the items be put into the bags if the items are identical?

    Would this be a problem of placing indistinguishable items into distinguishable boxes?
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    Re: Putting items in bags

    I used this theorem from my book regarding "Indistinguishable objects and distinguishable boxes": C(n + r - 1, r).
    We have 20 indistinguishable books going in 12 distinguishable bags. Let n = 12 and r = 20. We have C(12 + 20 - 1, 20) = C(31, 20) different ways place the items.

    I'm not sure if this is correct or not. Suggestions?
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    Re: Putting items in bags

    Quote Originally Posted by battery88 View Post
    I used this theorem from my book regarding "Indistinguishable objects and distinguishable boxes": C(n + r - 1, r).
    We have 20 indistinguishable books going in 12 distinguishable bags. Let n = 12 and r = 20. We have C(12 + 20 - 1, 20) = C(31, 20) different ways place the items.

    You have read the textbook incorrectly.
    "N indistinguishable objects and K distinguishable boxes"
    \binom{N+K-1}{N}=\binom{N+K-1}{K-1}.

    So 20 indistinguishable books going in 12 distinguishable bags gives
    \binom{20+12-1}{20}
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    Re: Putting items in bags

    Okay, thanks for the correction.
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    Re: Putting items in bags

    Now, one more question going back to Plato's post 3:

    Suppose there are 20 items and 12 bags. How many ways can the items be put into the bags if the items are identical and no bag can be left empty?
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  12. #12
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    Re: Putting items in bags

    Quote Originally Posted by battery88 View Post
    Suppose there are 20 items and 12 bags. How many ways can the items be put into the bags if the items are identical and no bag can be left empty?
    You may find this webpage useful.

    If the 20 items are identical but the bags are distinct go ahead and put one item in each bag. Now none is empty. There are eight items left, so:
    \binom{8+12-1}{8}.
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    Re: Putting items in bags

    Thanks for the link and the help. It seems so simple now. Just wish I could do it without help.
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