Question: what is the greatest common factor of 56 and 21?
Answer: The prime factorization of 56 is: 2 x 2 x 2 x 7.
The prime factorization of 21 is: 3 x 7.
The prime factors and multiplicities 56 and 21 have in common are: 7.
7 is the gcf of 56 and 21.
gcf(56,21) = 7.