How do you figure out a prime factor?

Answer 1

Most of the numbers you encounter are divisible by some combination of 2,3,5 and/or 7. Start there. If the number is even, keep dividing by two until it isn't. Then try three. And so on. Some people find factor trees helpful. The most important thing is that when you're done, all the factors are prime.

Answer 2

Keep dividing until the numbers can't reduce any more.

72

36,2

18,2,2

9,2,2,2

3,3,2,2,2

2 and 3 are prime, the only prime factors of 72.

2 x 2 x 2 x 3 x 3 = 72