Question: what is the greatest common factor of 72 and 45?
Answer: The prime factorization of 45 is: 3 x 3 x 5.
The prime factorization of 72 is: 2 x 2 x 2 x 3 x 3.
The prime factors and multiplicities 45 and 72 have in common are: 3 x 3.
3 x 3 is the gcf of 45 and 72.
gcf(45,72) = 9.