Prime and composite numbers
Name: _____________________ Date: __________
Show your thinking with numbers, words, or a drawing.
1. Which of these numbers is prime? 25, 37, 39, 1
2. Which of these numbers is prime? 43, 1, 27, 49
3. Which of these numbers is prime? 39, 1, 25, 47
4. Which of these numbers is prime? 35, 23, 1, 39
5. Which of these numbers is prime? 7, 21, 1, 39
Answer key · Prime and composite numbers
1. 37
A prime number has exactly two factors: 1 and itself. 1 has only one factor, so it is not prime.
25 = 5 × 5 and 39 = 3 × 13, so both are composite.
37 has only the factors 1 and 37, so 37 is prime.
2. 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 49 = 7 × 7, so both are composite.
43 has only the factors 1 and 43, so 43 is prime.
3. 47
A prime number has exactly two factors: 1 and itself. 1 has only one factor, so it is not prime.
25 = 5 × 5 and 39 = 3 × 13, so both are composite.
47 has only the factors 1 and 47, so 47 is prime.
4. 23
A prime number has exactly two factors: 1 and itself. 1 has only one factor, so it is not prime.
35 = 5 × 7 and 39 = 3 × 13, so both are composite.
23 has only the factors 1 and 23, so 23 is prime.
5. 7
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.
Try it together
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