How do you find the greatest common factor lof 21 n 630?

Answer 1

You list all the factors of each number, then compare the lists to find numbers that

are on both lists, then finally pick the highest of those.

Factors of 21: 1, 3, 7, and 21.

Factors of 630:

1, 2, 3, 5, 6, 7, 9, 10, 15, 18, 21, 30, 35, 42, 63, 70, 90, 105, 126, 210, 315, and 630.

Numbers on both lists (common factors): 1, 3, 7, and 21.

Highest common factor: 21

Answer 2

Since 21 is a factor of 630, it is automatically the GCF.