Question: what is the greatest common factor between 21 and 42?
Answer: The prime factorization of 21 is: 3 x 7.
The prime factorization of 42 is: 2 x 3 x 7.
The prime factors and multiplicities 21 and 42 have in common are: 3 x 7.
3 x 7 is the gcf of 21 and 42.
gcf(21,42) = 21.