To find the GCF, you need to list prime numbers that can multiply into 42 and 70.
42 - 7 * 2 * 3
70 - 7 * 2 * 5
So, the GCF, or Greatest Common Factor, of 42 and 70 is (7 * 2), since you can use the prime numbers 7 and 2 in both of their factor lists.
