GCF is the biggest multiple that fits into both numbers. The factors of 20 are (2, 2, 5) and the factors of 35 are (5, 7). The largest number in both numbers is 5. Therefore, 20+35 can be rewritten as 5(4+7), since you divide 20 and 35 by the GCF (5). Factors of 64 are (2, 2, 2, 2, 2, 2) and factors of 40 are (2, 2, 2, 5). Both numbers have (2, 2, 2) as factors. If you multiply 2*2*2, then you get 8. The GCF is 8. Therefore, 40+64 can be rewritten as 8(8+5), since you divide 64 and 40 by the GCF (8).