Counting problem.

  • Thread starter gimpy
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  • #1
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I think i got this answer.

How many ways are there to distribute five distinguishable objects into three indistinguishable boxes?

Wouldn't the answer just be [tex]C(5,3)[/tex] because the boxes are indistinguishable? Or do i treat this question the same as if the boxes were distinguishable?
 

Answers and Replies

  • #2
selfAdjoint
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The binomial coefficient [tex]C_3^5[/tex] is right. You could do it as if the boxes were distinguishable and then divide by the number of sequences of them: 3! = 6. and that's what you would get.
 
  • #3
Hurkyl
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Multiple objects can fit in a box, can't they?
 
  • #4
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Unless they are very, very big.
 
  • #5
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Originally posted by Hurkyl
Multiple objects can fit in a box, can't they?
Yes, multiple objects can fit into a box.

I think my answer is correct, isn't it?
 
  • #6
matt grime
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I don't think it is. I think it's quite a way out. The answer is the same as the number of ways to partition 5 into three (possibly empty?) subsets, which is

[tex]\sum\binom{n}{p}\binom{n}{q}\binom{n}{r}[/tex]

for all p+q+r = 5 (and possibly with the requirement that p,q,r are strictyl positive.

To show that your answer is wrong, I think your reasoning that the number of ways to put 5 distinct objects into 1 box is 5 choose 1, ie 5, when obviously it is 1. Also there are many ways of putting 5 balls into 100 boxes, and 5 choose 100 is define to be 0.
 
  • #7
NateTG
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There are 5 possible groupings:
0 0 5 - 1
0 1 4 - 5
0 2 3 - 10
1 1 3 - 10
1 2 2 - 20

For a total of 46 possibilities. I don't see a more general formula for this.
 
  • #8
matt grime
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The answer is the partition number of (5,3) for want of a better term which is what you've worked out, NateG, and does not follow the formula I wrote just there. Anyway, it isn't 5 choose 3. Is
[tex]\sum\binom{n}{p}\binom{n-p}{q}\binom{n-p-q}{r}[/tex]

getting better? I think so.

matt
 
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