Question: what is a greatest common factor of 80 and 20?
Answer: The prime factorization of 20 is: 2 x 2 x 5.
The prime factorization of 80 is: 2 x 2 x 2 x 2 x 5.
The prime factors and multiplicities 20 and 80 have in common are: 2 x 2 x 5.
2 x 2 x 5 is the gcf of 20 and 80.
gcf(20,80) = 20.