What are the common factors of 21 and 66?

The common factors of 21 and 66 are 1 and 3.

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

The factors of 66 are 1, 2, 3, 6, 11, 22, 33, and 66.

The common factors are 1 and 3.

The greatest common factor is 3.