What is the greatest common factor of 21 and 30?

The GCF of 21 and 30 is 3

Definition: A factor is a divisor - a number that will evenly divide into another number. The greatest common factor of two or more numbers is the largest factor that the numbers have in common.

Method:

One way to determine the common factors and greatest common factor is to find all the factors of the numbers and compare them.

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

The factors of 30 are 1, 2, 3, 5, 6, 10, 15, and 30.

The common factors are 1 and 3. Therefore, the greatest common factor is 3.