What are the common factors of 12 and 21?

The common factors of 12 and 21 are 1 and 3.

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 12 are 1, 2, 3, 4, 6, and 12.

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

The common factors are 1 and 3.

This means that the greatest common factor is 3.

The numbers are 1 and 3