NCERT Solutions for Ganita Prakash Chapter 5 Prime Time
Updated on 2026-09-05
About this chapter
A multiple of a number is what you get on multiplying it by 1, 2, 3, … A factor (or divisor) of a number divides it exactly, leaving remainder 0. Numbers that are multiples of both numbers are their common multiples ; numbers that divide both are their common factors . A prime number has exactly two factors — 1 and itself (2, 3, 5, 7, 11, …). A composite number has more than two factors (4, 6, 8, 9, …). The number 1 is neither prime nor composite , as it has only one factor. The Sieve of Eratosthenes lists primes by circling a number and crossing out all its later multiples. It gives exactly 25 primes below 100 . Two numbers with no common factor other than 1 are co-prime — 4 and 9, 17 and 69, 40 and 231. Any two different primes are always co-prime. Every number greater than 1 can be writ
- Common Multiples and Common Factors
- & 109Playing with other number pairs
- Jump Jackpot
- & 111Section 5.1 Common Multiples and Common Factors
- Prime Numbers
- & 115Section 5.2 Prime Numbers
- Co-prime numbers for safekeeping treasures
- Prime Factorisation
- Using prime factorisation
- Divisibility Tests
Exercises
- In-text Question — Common Multiples and Common Factors Page 107
- Figure it Out — Common Multiples and Common Factors Page 108
- & 109Playing with other number pairs — In-text Questions Page 108
- Jump Jackpot — In-text Questions Page 110
- & 111Section 5.1 Common Multiples and Common Factors — Figure it Out Page 110
- In-text Questions — Prime Numbers Page 112 to 114
- & 115Section 5.2 Prime Numbers — Figure it Out Page 114
- In-text Questions — Co-prime numbers for safekeeping treasures Page 115 to 117
- In-text Questions — Prime Factorisation Page 117 to 120
- Figure it Out — Prime Factorisation Page 120
- Using prime factorisation — Figure it Out Page 122
- In-text Questions — Divisibility Tests Page 123 to 125
- & 126Section 5.5 Divisibility Tests — Figure it Out Page 125
- In-text Questions — Fun with numbers Page 126 to 128