First, you have to do some factorization
60 = {1,2,3,4,5,6,10,12,15,20,30,60}
72 = {1,2,3,4,6,8,9,12,18,24,36,72}
the GCF is 12
now we find the number that you multiply by 12 to get 60 and another number to get 72.
12 x 5 = 60
12 x 6 = 72
now we notice if you add 60 + 72, we can now tell that it also equals (12)(5)+(12)(6)= 12(5+6)
