The greatest common factor (GCF) of 56 and 21 is: 7
To find the common factors between them, you must only find the factors that go up to the lowest number which in this case would be 21.
Factors of 21: 1, 3, 7, 21
Factors of 56: 1, 2, 4, 7, 8, 14
The common factors between the two numbers are 1 and 7. We are looking for the greatest common factor, which would be 7 since it is the highest.