Number System

What you'll learn

A concept-first guide to the Number System for SSC-CGL: how numbers are classified, must-know divisibility rules, key sum formulas, the unit-digit (cyclicity) trick with a worked example, and a quiz to test yourself.

This chapter has

Number System — solved practice questions

8 SSC / Banking Quantitative Aptitude questions with step-by-step solutions. Attempt each, then reveal the answer.

1. Q1easy

   Which of the following is NOT a prime number?

   • A91
   • B97
   • C89
   • D83
   Show answer & solution

   Correct answer: (A) 91

   91 = 7 × 13, so it is not prime. 97, 89 and 83 are all prime.

2. Q2easy

   The LCM of 12, 18 and 24 is:

   • A48
   • B60
   • C72
   • D144
   Show answer & solution

   Correct answer: (C) 72

   12 = 2²×3, 18 = 2×3², 24 = 2³×3. LCM = 2³×3² = 72.

3. Q3easy

   The HCF of 48 and 72 is:

   • A12
   • B24
   • C36
   • D48
   Show answer & solution

   Correct answer: (B) 24

   48 = 2⁴×3, 72 = 2³×3². HCF = 2³×3 = 24.

4. Q4medium

   What is the unit digit of 7⁴⁰?

   • A7
   • B9
   • C1
   • D3
   Show answer & solution

   Correct answer: (C) 1

   Powers of 7 cycle: 7,9,3,1,7,9,3,1 (period 4). 40÷4=10 remainder 0 → unit digit = 1.

5. Q5medium

   The sum of all prime numbers between 1 and 20 is:

   • A58
   • B67
   • C77
   • D87
   Show answer & solution

   Correct answer: (C) 77

   Primes: 2,3,5,7,11,13,17,19. Sum = 77.

6. Q6easy

   A number is divisible by 9 if:

   • Asum of digits divisible by 9
   • Blast digit is 9
   • Clast two digits divisible by 9
   • Dnumber ends in 0
   Show answer & solution

   Correct answer: (A) sum of digits divisible by 9

   A number is divisible by 9 if the sum of its digits is divisible by 9.

7. Q7hard

   What is the remainder when 7²³ is divided by 48?

   • A1
   • B7
   • C23
   • D41
   Show answer & solution

   Correct answer: (B) 7

   7¹=7, 7²=49≡1(mod 48). So 7²³=(7²)¹¹×7≡1¹¹×7=7.

8. Q8medium

   The product of two numbers is 2160 and their HCF is 12. Their LCM is:

   • A120
   • B180
   • C240
   • D360
   Show answer & solution

   Correct answer: (B) 180

   LCM = Product/HCF = 2160/12 = 180.

Want the full set? Practise all 15 Number System questions →