Need formal proofs please.

Definition: Given any nonnegative integer n, the decimal representation of n is an expression of the form: dkdk-1 ...dzd,do. where k is a nonnegative integer; do.di,d. dk (called the decimal digits of n) are integers from 0 to 9 inclusive; dk 0 unless n = 0 and k 0; and n = dy 10* + di-1 10*-1++ d2 102 + d, - 10 + do For example 2,503 2-103 + 5-102 + 0-10 + 3. (18 pts, 3pts for a and 5 pts each for the rest) 7. Prove the following statements (a) For all integers a, b, and c, if alb and alc then al(b + c) and al(b -c). (b) Prove that if n is any nonnegative integer whose decimal representation ends in 0 or 5, then 5In. 4ld1 10 + do, then 4In. then n is divisible by 3. (c) Prove that if the decimal representation of a nonnegative integer n ends in dydg and if (d) Prove that for any nonnegative integer n, if the sum of the digits of n is divisible by 3