Instructions
A prime number is a whole number greater than 1 that has only two factors: 1 and itself.
Examples: 2, 3, 5, 7, 11, 13
Prime factors are the prime numbers that you multiply together to get a specific number. A simple way to find them is by using a factor tree.
- Start with your number at the top.
- Draw two "branches" down from the number and write any two factors that multiply to make that number.
- Look at your new numbers. If a number is prime, circle it. This branch is finished.
- If a number is not prime, draw two new branches below it and write two of its factors.
- Continue this process until all the numbers at the ends of the branches are circled prime numbers.
- Write all the circled numbers in a multiplication sentence to get the prime factorization.
Example: Find the prime factors of 20
Start with 20. Two factors are 4 and 5.
The number 5 is prime, so we circle it.
The number 4 is not prime. Its factors are 2 and 2.
The number 2 is prime, so we circle both 2s.
The circled prime numbers are 2, 2, and 5.
The prime factorization of 20 is 2 × 2 × 5.
Practice Problems
Find the prime factors for each number below. Write your final answer on the line.
| 1. 12 = | 2. 18 = |
| 3. 27 = | 4. 30 = |
| 5. 45 = | 6. 56 = |
| 7. 72 = | 8. 100 = |
Answer Key
- 12 = 2 × 2 × 3
- 18 = 2 × 3 × 3
- 27 = 3 × 3 × 3
- 30 = 2 × 3 × 5
- 45 = 3 × 3 × 5
- 56 = 2 × 2 × 2 × 7
- 72 = 2 × 2 × 2 × 3 × 3
- 100 = 2 × 2 × 5 × 5