number · Year 6 · AC9M6N02
Prime and composite numbers
A prime number has exactly two factors: 1 and itself, like 2, 3, 5, 7 and 11. A composite number has more than two factors, like 12. The number 1 is neither, because it has only one factor. 2 is the only even prime. Being odd does not make a number prime: 91 = 7 × 13. To test a number, try dividing by 2, 3, 5 and 7.
Sample questions
Which of these numbers is prime? 7, 21, 39, 1
- A prime number has exactly two factors: 1 and itself. 1 has only one factor, so it is not prime.
- 21 = 3 × 7 and 39 = 3 × 13, so both are composite.
- 7 has only the factors 1 and 7, so 7 is prime.
Answer: 7
Which of these numbers is prime? 49, 1, 43, 45
- A prime number has exactly two factors: 1 and itself. 1 has only one factor, so it is not prime.
- 45 = 3 × 15 and 49 = 7 × 7, so both are composite.
- 43 has only the factors 1 and 43, so 43 is prime.
Answer: 43
Which of these numbers is prime? 39, 27, 1, 43
- A prime number has exactly two factors: 1 and itself. 1 has only one factor, so it is not prime.
- 27 = 3 × 9 and 39 = 3 × 13, so both are composite.
- 43 has only the factors 1 and 43, so 43 is prime.
Answer: 43