# Using the pigeonhole principle to prove there is at least two groups of people whose age sums are the same.

In a room there are 10 people, none of whom are older than 100 (ages are given in whole numbers only) but each of whom is at least 1 year old. Prove that one can always find two groups of people (possibly intersection, but different) the sums of whose ages are the same.

I know we have to use the pigeon hole principle. But I don't know how to find the pigeons and pigeonholes. Can someone help me out?

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It seems as though if you can solve it with possible intersection, you can solve it without intersection: If you remove the same person from two sets whose sum of ages are the same, the sum of ages stays the same. – Jason DeVito Dec 9 '12 at 3:29

@kevlar: Almost. That’s the number of non-empty subsets; there are $2^{10}=1024$ subsets altogether, including $\varnothing$, and nothing in the problem rules it out. (Of course it’s also of no use, since its sum of $0$ can’t be duplicated by any of the non-empty subsets, but it does count as one of the pigeons.) – Brian M. Scott Dec 9 '12 at 3:37
@kevlar: Yes; the possible sums range from $0$ through $1000$, so how many of them are there? – Brian M. Scott Dec 9 '12 at 4:04