Find the greatest common factor of

7, 15 and 21

7 is prime.

but

3 factors into 15 and 21

but

7 factors into 21

GCF = 1

for the values 15, 17, 21

Solution by Factorization:

The factors of 15 are: 1, 3, 5, 15

The factors of 17 are: 1, 17

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

Then the greatest common factor is 1.