Free practice on HCF and Prime Numbers for Class 5, following the Cameroon primary school curriculum (English subsystem). Every answer is explained, so a parent who is not sure of the answer can still help.
Every whole number > 1 is either prime or can be broken into primes. 12 = 2 × 2 × 3. 30 = 2 × 3 × 5. This is called prime factorisation.
| Number | Prime factors |
|---|---|
| 6 | 2 × 3 |
| 12 | 2 × 2 × 3 |
| 18 | 2 × 3 × 3 |
| 30 | 2 × 3 × 5 |
| 100 | 2 × 2 × 5 × 5 |
HCF = Highest Common Factor. Method: prime-factorise each number, then multiply the primes they share. HCF of 12 (2·2·3) and 18 (2·3·3) shares one 2 and one 3, so HCF = 6.
Even: divisible by 2 (2, 4, 6…). Odd: not divisible by 2 (1, 3, 5…). Prime: exactly two factors, 1 and itself (2, 3, 5, 7, 11, 13…).
Primes below 50: 2 · 3 · 5 · 7 · 11 · 13 · 17 · 19 · 23 · 29 · 31 · 37 · 41 · 43 · 47
What is the HCF of 12 and 18?
Write your answer, then check below.
Answer: 6
Factors of 12: 1,2,3,4,6,12. Factors of 18: 1,2,3,6,9,18. Highest common = 6.
What is the HCF of 20 and 30?
Write your answer, then check below.
Answer: 10
Which number is prime?
Answer: B. 17
17 can only be divided by 1 and 17.
Which of these is NOT a prime number?
Answer: C. 21
21 = 3 × 7, so it is not prime.
Write the smallest odd prime number.
Write your answer, then check below.
Answer: 3
2 is prime but even; 3 is the smallest odd prime.
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