Let's assume we were given 2 numbers: 15 and 30. Their sum is: We want to express it as the product of GCF and another sum. 15 is divisible by: 1, 3, 5, 15 30 is divisible by: 1, 2, 3, 5, 6, 10, 15, 30 The greatest number that appears in 2 series is 15. In this case sum of two numbers can always be written as: