I'm stuck on one of my homework problems:

Prove that if the decimal representation of a nonnegative integer n ends in d1d0 and if 4 | (10d1 + d0), then 4 | n.

It gave me a hint that if the decimal representation of a nonnegative integer n ends in d1d0, then there is an integer s such that n = 100s + 10d + d0

Also just wanted to check if this problem I did is correct:

For all integers a and b, if a|b then a|10 or a|b.

I used a counterexample of a=2 and b=3. Would that work?

One more thing, where or how can I make a source code out of my problems so they will be easier to read? Can't seem to find it in the editing tool or do I have to make them myself?

Thanks