# How do you find the greatest common factor of 9, 21?

Nov 4, 2016

$G C F \left(9 , 21\right) = 3$

#### Explanation:

factors 9:--$\left\{1 , 3 , 9\right\}$

factors 21:--$\left\{1 , 3 , 7 , 21\right\}$

common factors:---$\left\{1 , 3 , 9\right\} \cap \left\{1 , 3 , 7 , 21\right\}$

$= \left\{1 , 3\right\}$

$G C F \left(9 , 21\right) = 3$