What is the greatest common factor for 18 and 81?

The factors of 18 are 1, 2, 3, 6, 9, and 18.

The factors of 81 are 1, 3, 9, 27, and 81.

The common factors are 1, 3, and 9. Therefore, the greatest common factor is 9.

You can also determine the greatest common factor from the prime factors.

The prime factors of 18 are 2, 3, and 3.

The prime factors of 81 are 3, 3, 3, and 3.

The prime factors in common are 3 and 3, so the greatest common factor is 3 x 3 = 9.