What is the greatest common factor of 15 21 and 27?

Answer: 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 15 are 1, 3, 5, and 15.

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

The factors of 27 are 1, 3, 9, and 27.

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