Divisibility by
•A number is divisible by 9 if the sum of its digits is divisible by 9.
•Is 369 divisible by 9?
•369 is divisible by 9
•           because
•   3 + 6 + 9 =18
•       1 + 8 = 9
•              AND
•
•     9 is divisible by 9.