let n be contained in natural numbers and n=amam-1...a2a1a0 where each ai is a digit of n. Show that 3 divides n if and only if 3 divides (am+am-1+...+a2+a1+a0). Hint: write n as a base 10 expression in terms of sum of its digits and use number 4.

This is a homework problem for my discrete math class but it is not graded so we are allowed to work with others on it.

I do not even know where to start so could someone help me I do not understand at all.