![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/step1.JPG\")
Method 1:
\List the factors of each number.
\Factors of 21: 3, 7
\Factors of 45: 3, 3, 5
\Identify the greatest common factor.
\The greatest common factor of 21 and 45 is 3.
\![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/step2.JPG\")
Method 2:
\Write the prime factorization of each number.
\21 = 3 × 7.
\45 = 3 × 15.
\The GCF is the product of the common prime factors in each number.
\GCF = 3.
\The greatest common factor of 21 and 45 is 3.
\![\"\"](\"http://www.mathskey.com/userfiles/image/steps_images/solution.JPG\")
The greatest common factor of 21 and 45 is 3.