What is the greatest common factor of 15 and 39?
The factors of 15 are 1, 3, 5, and 15.
The factors of 39 are 1, 3, 13, and 39.
The factors they have in common are 1 and 3. Therefore, the
greatest common factor is 3.
You can also determine the greatest common factor from their
prime factors.
The prime factors of 15 are 3 and 5.
The prime factors of 39 are 3 and 13.
The prime factors they have in common are a single 3, so the
greatest common factor is 3.