What is the greatest common factor of 21 and 35?

The GCF of 21 and 35 is 7.

The factors of 21 are 1, 3, 7, and 21

The factors of 35 are 1, 5, 7, and 35

The greatest (largest) factor common to both 21 and 35 is 7
The GCF is 7.