Question: what is the greatest common factor 12 and 66?
Answer: The prime factorization of 12 is: 2 x 2 x 3.
The prime factorization of 66 is: 2 x 3 x 11.
The prime factors and multiplicities 12 and 66 have in common are: 2 x 3.
2 x 3 is the gcf of 12 and 66.
gcf(12,66) = 6.