What is the GFC of 21, 35, and 49?

The greatest common factor of 21, 35, and 49 is: 7

Since 21 is the lowest number we are solving up to, so no other factors can be given after that.

Factors of 21: 1, 3, 7, 21

Factors of 35: 1, 5, 7

Factors of 49, 1, 7

The common factors between the three numbers are 1 and 7, but we are looking for the greatest common factor, not the common ones. It either has to be 1 or 7, and 7 is obviously the highest, making it the greatest common factor.