Example: The common factors of 15 and 30
Factors of 15 are 1, 3, 5, and 15
Factors of 30 are 1, 2, 3, 5, 6, 10, 15 and 30
The factors that are common are 1, 3, 5 and 15
What is the "Greatest Common Factor" ?
It is simply the largest of the common factors.
The largest of the common factors is 15, so the Greatest Common Factor of 15 and 30 is 15