Thread: 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

have a look at permutations, it is really straightforward, take a look and have an attempt I will show you the answer then

Originally Posted by battery88

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:
.
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:
.

BUT, I don't think it means that the bags are distinct.

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.

Originally Posted by Plato

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:
.
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:
.

BUT, I don't think it means that the bags are distinct.

I was thinking of something else: placing distinguished items in distinguished boxes.

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

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?

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?

Originally Posted by battery88

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"
.

So 20 indistinguishable books going in 12 distinguishable bags gives

Okay, thanks for the correction.

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?

Originally Posted by battery88

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:
.

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