Definitions:

             The Greatest Common Factor, GCF, of two are more whole numbers is the largest
           whole number that is a factor of both (all) of the numbers and is written as GCF(a,b)
           where a and b are non-zero whole numbers

Find the GCF: Prime Factorization Method: see examples for other methods
             Step 1: Find the prime factors for each number
             Step 2: The GCF is the prime factors common to all numbers

    Examples:
            Find GCF(36, 48)
            Prime factors of 36: 2 x 2 x 3 x 3
            Prime factors of 48: 2 x 2 x 2 x 2 x 3
            So common factors to both is 2 x 2 x 3 = 12

            Then GCF(36, 48) is 12, i.e, 12 is the smallest factor that divides both 36 and 48

        Find GCF(390, 546)
            Prime factors 390: 2 x 3 x 5 x 13
            Prime factors 546: 2 x 3 x 7 x 13
            So common factors to both is 2 x 3 x 13 = 78

            Then GCF(390, 546) = 78