What is the greatest common factor of 21 and 60?

THe GCF of 21 and 60 is 3

21 is 3 * 7

60 is 2 * 2 * 3 * 5

What factors do they have in common? Just 3.

So 3 is the greatest common factor.
The GCF of 21 and 60 is 3.