Question

The number C=13 x 72 + 26. Without computing the value of C, explain whether or not C is divisible by 13.

Answer 1

This number is divisible by 13 because it's a sum of two numbers each of which is divisible by 13:
13 x 72 is divisible because it has 13 as a factor
26=2*13 therefore it's also divisible by 13