The steps for finding the common factors of 12 and 15

Rule #1: Find all of the numbers that multiply to 12 (1,2,3,4,6,12). 

Rule #2: After you find all the multiples of 12 you find all the multiples of 15 (1,3,5,15).

Rule #3: After you have found the multiples of 12 and 15 find the numbers that are the same from the multiples of 12 and 15 (1 and 3)

So, that means that 1 and 3 are the common factors of 12 and 15. That's all of the rules for finding the common factors of two numbers.