Class 6 Maths Chapter 5 Prime Time Revision Notes

Chapter 5 is built around one idea — how numbers relate to each other through factors and multiples — and this recap covers primes, composites, and co-primes at a glance.

Last Updated: September 23, 2026

Key Terms

• Factor: a number that divides another number exactly.
• Multiple: the result of multiplying a number by a whole number.
• Prime number: exactly two factors (1 and itself). 2 is the only even prime.
• Composite number: more than two factors.
• 1 is neither prime nor composite (only one factor).
• Co-prime numbers: only common factor is 1 (need not be prime themselves, e.g. 8 & 9).
• Twin primes: two primes differing by exactly 2 (e.g. 11, 13).

Key Lists

• Primes up to 100: 25 total, from 2 to 97.
• Twin prime pairs up to 100: (3,5), (5,7), (11,13), (17,19), (29,31), (41,43), (59,61), (71,73).
• Smallest number with 3 distinct prime factors = 30 (2×3×5); with 4 = 210 (2×3×5×7).

Divisibility Rules (this chapter covers 2, 4, 5, 8, 10 only)

Note: rules for 3, 6, 7, 9, and 11 are covered in later classes, not this chapter.

Prime Factorisation

• Every composite number has a unique prime factorisation (found via a factor tree).
• Two numbers are co-prime exactly when they share no common prime factors.
• If two numbers are co-prime, their product equals their LCM (common multiple relationship).

Quick revision: Solutions | Extra Questions for this chapter.

Frequently Asked Questions

How can you determine whether a given number is a prime number?
A number is prime if it has exactly two factors, 1 and itself, checked by testing whether any number other than 1 and itself divides it evenly.

What is a prime factorisation, and why is it useful?
Prime factorisation is expressing a number as a product of only its prime factors, useful for finding the Highest Common Factor and Lowest Common Multiple, as well as simplifying fractions.

Written by Satish

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