Discrete Mathematics

Is anyone knowledgeable in discrete? Preferably proofs, divisibility, congruency relations, and summations? I have an exam Friday and it would be great if someone could help me out or "dumb things down" for me because I am completely lost. Some of the problems are:

1. If the sum of the digits of a five digit number is divisible by 9, then the number is divisible by 9.
2. If m|n then m is less than or equal to n.
3. GCD(m,n) | n-m

I would appreciate any help!