91 five-digit numbers are written on a blackboard. Prove that one can find 3 numbers on the blackboard such that the sums of their digits are equal.

I was thinking of writing the three numbers as , and , but how do I proceed? Also, I know that there are different 5-digit numbers possible, but is that information going to help? This doesn't look like a permutations/combinations question to me...

Please help immediately. It's for a friend... and my mathematical conscience is pricking me bad...

Thanks!

ILoveMaths07