Question: what is a greatest common factor of 72 and 28?
Answer: The prime factorization of 28 is: 2 x 2 x 7.
The prime factorization of 72 is: 2 x 2 x 2 x 3 x 3.
The prime factors and multiplicities 28 and 72 have in common are: 2 x 2.
2 x 2 is the gcf of 28 and 72.
gcf(28,72) = 4.