Question: what is the greatest common factor between 18 and 21?
Answer: The prime factorization of 18 is: 2 x 3 x 3.
The prime factorization of 21 is: 3 x 7.
The prime factors and multiplicities 18 and 21 have in common are: 3.
3 is the gcf of 18 and 21.
gcf(18,21) = 3.