Greatest Common Factor Find it using a Venn Diagram Sometimes, it is very easy to find the Greatest Common Factor of 2 numbers. For example, what is the Greatest Common Factor of 15 and 10? 5 Greatest Common Factor Most of the time, however, it is a little more difficult, and so today we are going to take some notes together on a great method to find the Greatest Common Factor of 2 numbers. Sooooooo, please take out a blank sheet of paper, and get ready to take some notes. Greatest Common Factor Example: Find the GCF of 54 and 36 STEP 1: Use a factor tree to find the prime factorization of both numbers. 54 36 9 3 3 6 3 12 2 4 2 3 3 2 Finding the Greatest Common Factor – Step 1. ◦ The prime factorization of 54 is: ◦2 x 3 x 3 x 3 ◦ The prime factorization of 36 is: ◦3 x 3 x 2 x 2 Finding the Greatest Common Factor – Step 1. Step 2: Create a Venn Diagram (2 overlapping circles). For each pair of common prime factors, put one of those in the middle. Finding the Greatest Common Factor – Step 2. Example: 3 3 2 Finding the Greatest Common Factor – Step 2. Step 3: Once you have put all of the pairs in the middle, multiply them together. So, we need to multiply 3 x 3 x 2 The Greatest Common Factor of 54 and 36 is 18 Finding the Greatest Common Factor – Step 3.