![\"\"](\"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 42: 2, 3, 6, 7, 14, 21, 42
\Identify the greatest common factor.
\The greatest common factor of 21 and 42 is 21.![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/step2.JPG\")
Method 2:
\Write the prime factorization of each number.
\21 = 3 × 7.
\42 = 2 × 3 × 7.
\The GCF is the product of the common prime factors in each number.
\GCF = 3 × 7 = 21
\The greatest common factor of 21 and 42 is 21.![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/solution.JPG\")
The greatest common factor of 21 and 42 is 21.