Formula for common factors

Answer 1

There isn't one. You need to know the multiplication tables, and think how they apply to each number.

example: 24 and 36

both belong to the 3's mult. table so 3 is a common factor (3x8 and 3x12)

but they also belong to the 4's Mult table so 4 is a common factor (4x6 and 4x9)

but they also belong to the 12's Mult table so 12 is a common factor (2x12 and 3x12)

3, 4, and 12 are common factors, but the GCF is 12