Someone asked me this question a few days ago, and I have a few ideas on how to do it, but it seems to be too much work for a problem of this nature.

We know that a number is divisible by 3 if the sum of all its digits are divisible by 3.

The question is either:

Prove that this is so

or

Prove why this is so

I don't remember, but I think they amount to pretty much the same thing

Thanks guys