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 $\displaystyle 10000a + 1000b + 100c + 10d + e $, $\displaystyle 10000f + 1000g + 100h + 10i + j $ and $\displaystyle 10000k + 1000l + 100m + 10n + o $, but how do I proceed? Also, I know that there are $\displaystyle 9*10^{4} $ 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