What are the common factors of 20 32 and 72?

The factors of 20 are:

1, 2, 4, 5, 10, 20

The factors of 32 are:

1, 2, 4, 8, 16, 32

The factors of 72 are:

1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72

The common factors are:

1, 2, 4