Make sure you give AL2006 a medal First of all, as you read the problem, it should begin to dawn on you that you'll need to know their greatest common factor. So let's find that: The factors of 56 are 1, 2, 4, 7, 8, 13, 28, and 56 . The factors of 64 are 1, 2, 4, 8, 16, 32, and 64 . The common factors of 56 and 64 are 1, 2, 4, and 8 . The greatest one is 8 . 56 = 8 x 7 64 = 8 x 8
So the sum of (56 + 64) is the sum of (8 x 7) + (8 x 8).
You can 'factor' 8 out of the terms of that sum, and get