What is the common factor of 16 and 20?

The factors of 16 are 1, 2, 4, 8, and 16.

The factors of 20 are 1, 2, 4, 5, 10, and 20.

The largest of the factors they have in common is 4, so 4 is the greatest common factor.

Using prime factorization:

The prime factors of 16 are 2, 2, 2, and 2.

The prime factors of 20 are 2, 2, and 5.

The prime factors in common are 2 and 2, so the greatest common factor is 2 x 2 = 4.