![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/step1.JPG\")
Method 1:
\List the factors of each number.
\Factors of 21 : 3, 7, 21
\Factors of 99 : 3, 9, 11, 33, 99
\Identify the greatest common factor.
\The greatest common factor of 21 and 99 is 3.![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/step2.JPG\")
Method 2:
\Write the prime factorization of each number.
\21 = 3 × 7.
\99 = 3 × 3 × 11.
\The GCF is the product of the common prime factors in each number.
\GCF = 3.
\The greatest common factor of 21 and 99 is 3.![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/solution.JPG\")
The greatest common factor of 21 and 99 is 3.