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Prime Numbers and Factors

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Prime Numbers and Factors

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📚 Part 1: Multiple Choice

1. Which number below is a prime number?

21

29

35

2. Which number below is a prime number?

51

59

63

3. Check all that apply: Which of the following are prime numbers?

2

9

17

4. Which of the following is NOT a prime number?

97

91

89

✏️ Part 2: Short Answers — HCF and LCM

5. List all prime numbers between 40 and 60.
6. Find the Highest Common Factor (HCF) of 48 and 84. Show your working.
7. Find the Highest Common Factor (HCF) of 63 and 81. Show your working.
8. Find the Least Common Multiple (LCM) of 6 and 8. Show your working.
9. Find the Least Common Multiple (LCM) of 7 and 5.
10. In one sentence, explain how you use prime factorisation to find the HCF of two numbers.

🔑 Answer Key

1. Which number below is a prime number?

Answer: 29 (only has factors 1 and 29)

2. Which number below is a prime number?

Answer: 59 (only has factors 1 and 59)

3. Check all that apply: Which of the following are prime numbers?

Answer: 2 and 17 (2 is the only even prime number; 17 only has factors 1 and 17; 9 = 3 × 3)

4. Which of the following is NOT a prime number?

Answer: 91 (91 = 7 × 13)

5. List all prime numbers between 40 and 60.

Answer: 41, 43, 47, 53, 59

6. Find the Highest Common Factor (HCF) of 48 and 84.

Answer: HCF = 12

Working: 48 = 2⁴ × 3, 84 = 2² × 3 × 7. HCF = 2² × 3 = 12

7. Find the Highest Common Factor (HCF) of 63 and 81.

Answer: HCF = 9

Working: 63 = 3² × 7, 81 = 3⁴. HCF = 3² = 9

8. Find the Least Common Multiple (LCM) of 6 and 8.

Answer: LCM = 24

Working: 6 = 2 × 3, 8 = 2³. LCM = 2³ × 3 = 24

9. Find the Least Common Multiple (LCM) of 7 and 5.

Answer: LCM = 35 (since 7 and 5 are both prime and have no common factors)

10. In one sentence, explain how you use prime factorisation to find the HCF of two numbers.

Answer: To find the HCF using prime factorisation, multiply together all the common prime factors using the lowest power that appears in both numbers.

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