How do you find the greatest common factor of 15 and 20?

Answer 1

-- List all the factors for each number:

Factors of 15: 1, 3, 5, and 15.

Factors of 20: 1, 2, 4, 5, 10, and 20.

-- Look over the lists. Find any numbers that are on both lists:

Common factors of 15 and 20: 1 and 5 .

-- Pick out the common factor that's biggest:

Greatest common factor of 15 and 20 is 5 .

Answer 2

3 x 5 = 15

2 x 2 x 5 = 20

Select the common factor.

The GCF is 5.