Question

A number is divisible by 3 if and only if the sum of its digits is divisible by 3.

Answer 1

For example 15 is divisible by 3 and 1+5=6 is divisible by 3.
What is another way to express a number abc that contains terms?

Answer 2

there are many, exp. divisible by 5 iff last digit is a 0 or 5

there is a cool one for 11, look it up.