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.