Class 6 Maths Chapter 5 Prime Time Extra Questions

Factors, multiples, prime numbers, and divisibility rules make up the backbone of Chapter 5, and this question set pushes a little further into co-primes and twin primes.

Last Updated: September 23, 2026

Very Short Answer Questions

Q1. What is the only even prime number?
Answer: 2.

Q2. Is 1 a prime number?
Answer: No — 1 has only one factor (itself), so it is neither prime nor composite.

Q3. Are 4 and 9 co-prime?
Answer: Yes — their only common factor is 1, even though neither number is itself prime.

Q4. What is the divisibility rule for 5?
Answer: A number is divisible by 5 if its last digit is 0 or 5.

Q5. What are twin primes?
Answer: Pairs of prime numbers that differ by exactly 2 (e.g., 11 and 13).

Short Answer Questions

Q6. List the first 10 prime numbers.
Answer: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29.

Q7. Find the prime factorisation of 360.
Answer: 360 = 2³ × 3² × 5.

Q8. Is 91 a prime number? Justify.
Answer: No — 91 = 7 × 13, so it has factors other than 1 and itself.

Q9. Find all common factors of 36 and 48.
Answer: Factors of 36: 1,2,3,4,6,9,12,18,36. Factors of 48: 1,2,3,4,6,8,12,16,24,48. Common factors: 1, 2, 3, 4, 6, 12.

Q10. Using the last-3-digits rule, check if 47128 is divisible by 8.
Answer: Last 3 digits = 128. 128 ÷ 8 = 16, so yes, 47128 is divisible by 8.

Long Answer / Reasoning Questions

Q11. Explain why every even number greater than 2 must be composite.
Answer: Every even number greater than 2 is divisible by 2, in addition to 1 and itself. That means it has at least three factors: 1, 2, and the number itself — which is more than the exactly-two factors required to be prime. So it must be composite.

Q12. Find the smallest number that has exactly four distinct prime factors, and write its prime factorisation.
Answer: Using the four smallest primes (2, 3, 5, 7): 2 × 3 × 5 × 7 = 210.

Q13. Two numbers are co-prime but neither of them is a prime number. Give one such example and explain why they are co-prime.
Answer: Example: 8 and 9. 8 = 2³ and 9 = 3² — they share no common prime factors, so their only common factor is 1, making them co-prime, even though neither 8 nor 9 is itself a prime number.

Q14. A number is divisible by both 4 and 5. What can you say about its divisibility by 20? Explain with an example.
Answer: Since 4 and 5 are co-prime (their only common factor is 1), any number divisible by both must also be divisible by their product, 20. Example: 60 is divisible by 4 (60÷4=15) and by 5 (60÷5=12), and indeed 60÷20=3, confirming it is divisible by 20 as well.

Q15. Explain, using prime factorisation, why 999 is not divisible by 99, even though 999 has “more” factors of 3 than 99 does.
Answer: 999 = 3³ × 37 and 99 = 3² × 11. For 999 to be divisible by 99, every prime factor of 99 (namely 3² and 11) would need to appear in 999’s own factorisation with at least the same power. While 999 does have enough factors of 3 (3³ covers 3²), it has zero factors of 11 — so the missing prime factor 11 is what actually blocks divisibility, not the count of 3s.

Practice more: Solutions | Revision Notes for this chapter.

Frequently Asked Questions

How would you find the Highest Common Factor of 18 and 24 using prime factorisation?
Breaking both numbers into prime factors, 18 equals 2 times 3 times 3, and 24 equals 2 times 2 times 2 times 3, then multiplying the common factors, 2 and 3, gives a Highest Common Factor of 6.

Why is 1 considered neither a prime number nor a composite number?
A prime number must have exactly two distinct factors and a composite number more than two, but 1 has only one factor, itself, so it does not satisfy either definition.

Written by Satish

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