This is a complete lesson for teaching your students how to find the prime factors of a number. There are rules for determining if a number can be divided by 2, 3, 5, or 7. The rule for 7 is complicated, so I cover it in Lesson 2. This Lesson covers the rules for 2, 3, and 5. Consider the number 390. • If a number is even, we can divide it by 2. 390 ÷ 2 = 195. The first prime factor is 2. • 195 is not even, but if we add up all the digits (1 + 9 + 5), we get 15, which is a multiple of 3, s