Question

Prove that a positive integer is divisible by 3 if and only if

Answer 1

Divisibility by Three. On this page we prove the theorem known from school that an integer is divisible by 3 if and only if the sum of its digits is divisible by 3. ... Hence all the numbers ak*bk are divisible by 3. Hence their sum (which is x-s) is divisible by 3.