First, find all the factors of the two numbers (separately): 72: 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, and 72 48: 1, 2, 3, 4, 6, 8, 12, 16, 24, 48 The numbers in bold are factors of both 72 and 48. Find the greatest one (out of the numbers in bold). 24 is the GCF.