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Prime Time – Class 6 Mathematics notes, important questions and MCQs

NCERT Class 6 Mathematics, chapter 5. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Prime numbers are the building blocks of all whole numbers. Factors, multiples and prime factorisation help solve many puzzles.

Factors and multiples

A factor divides a number exactly; a multiple is obtained by multiplying.
Every number is a factor and a multiple of itself; 1 is a factor of every number.
Common factors and common multiples of two numbers.

Primes

Prime: exactly two factors (2, 3, 5, 7, 11, 13…). Composite: more than two factors.
1 is neither prime nor composite; 2 is the only even prime.
Prime factorisation: 60 = 2 × 2 × 3 × 5.
Co-prime numbers have only 1 as a common factor (8 and 15).

Key terms

Prime number: number with exactly two factors
Composite number: number with more than two factors
Co-prime: two numbers with no common factor other than 1

Formulas

60 = 2² × 3 × 5

Important questions with answers

List all prime numbers between 20 and 40.2 marks

23, 29, 31, 37.

Is 1 a prime number? Is 2 a prime number? Explain.3 marks

1 is neither prime nor composite because it has only one factor. 2 is a prime because its only factors are 1 and itself; it is the only even prime.

Use the divisibility rules for 2, 3, 4, 5, 9 and 11 to test 7,128. Then find its prime factorisation.5 marks

7,128 ends in 8, so it is divisible by 2; last two digits 28 are divisible by 4, so divisible by 4. Digit sum 7 + 1 + 2 + 8 = 18, divisible by 3 and 9. It does not end in 0 or 5, so not divisible by 5. For 11: (7 + 2) − (1 + 8) = 0, so divisible by 11. 7,128 = 2³ × 3⁴ × 11 (8 × 81 × 11 = 7,128).

Quick revision

2 is the only even prime.
1 is neither prime nor composite.
Every composite number has a unique prime factorisation.

Exam tips

To test if a number is prime, check divisibility by primes up to its square root.
Twin primes differ by 2 (11 and 13).

Practice MCQs

Which is prime?
1. 9
2. 15
3. 17
4. 21
Show answer
Answer: (c) 17. Only factors 1 and 17.
The smallest prime number is
1. 0
2. 1
3. 2
4. 3
Show answer
Answer: (c) 2. 2.
Prime factorisation of 36 is
1. 2 × 18
2. 2² × 3²
3. 4 × 9
4. 6 × 6
Show answer
Answer: (b) 2² × 3². Only primes.
How many primes are there between 1 and 20?
1. 6
2. 7
3. 8
4. 9
Show answer
Answer: (c) 8. 2, 3, 5, 7, 11, 13, 17, 19.
Which pair is co-prime
1. 8 and 12
2. 15 and 25
3. 9 and 16
4. 14 and 21
Show answer
Answer: (c) 9 and 16. Only common factor is 1.
Which pair is co-prime?
1. 6, 9
2. 8, 15
3. 10, 25
4. 12, 18
Show answer
Answer: (b) 8, 15. HCF is 1.
Which is a pair of twin primes?
1. 7, 9
2. 21, 23
3. 17, 19
4. 13, 15
Show answer
Answer: (c) 17, 19. Both prime, differ by 2.
The prime factorisation of 180 is
1. 2² × 3² × 5
2. 2 × 3² × 10
3. 2³ × 3 × 5
4. 2² × 45
Show answer
Answer: (a) 2² × 3² × 5. 4 × 9 × 5.

All chapters of Class 6 Mathematics

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.