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.