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# What is the GCF of 21 and 63?

Oct 23, 2016

$21$

#### Explanation:

The GCF (greatest common factor) of two numbers is the greatest number which is a factor of both of them.

The factors of $21$ are: $1 , 3 , 7 , 21$

The factors of $63$ are: $1 , 3 , 7 , 9 , 21 , 63$

The greatest number which is on both lists is $21$, meaning $21$ is the greatest common factor.