Group D General Intelligence & Reasoning: Number Series — Concept Guide + MCQs

What is Number Series?

A number series is a sequence of numbers following a specific pattern. You must identify the rule and find the missing/next term.

Common Patterns

  • Arithmetic Series (AP): Constant difference — 2,5,8,11 (d=+3)
  • Geometric Series (GP): Constant ratio — 3,9,27,81 (r=×3)
  • Square Series: 1,4,9,16,25 (n²)
  • Cube Series: 1,8,27,64,125 (n³)
  • Fibonacci Series: 1,1,2,3,5,8,13 (each term = sum of two preceding)
  • Difference Series: Differences themselves form a pattern — 2,6,12,20 (differences: 4,6,8)
  • Mixed Operations: ×2+1, ×2–1, alternating patterns
  • Prime Number Series: 2,3,5,7,11,13...

Shortcut Approach

Step 1: Find first differences (term2–term1, term3–term2, etc.)
Step 2: If differences are constant → AP. If constant ratio → GP.
Step 3: If differences increase/decrease uniformly → second-order series.
Step 4: Check if terms are perfect squares/cubes ± constant.
Step 5: For large numbers, try dividing consecutive terms to check for GP.

Exam Tips

  • Memorise squares up to 20² = 400 and cubes up to 10³ = 1000
  • Prime numbers up to 50: 2,3,5,7,11,13,17,19,23,29,31,37,41,43,47
  • In RRB/Banking exams, 'wrong number in series' questions test the same skill

Practice MCQs

Q1. 2, 4, 6, 8, ?

  • A) 10
  • B) 11
  • C) 12
  • D) 9

Answer: A — Even numbers: +2 each time → 10.

Q2. 1, 3, 5, 7, ?

  • A) 9
  • B) 8
  • C) 10
  • D) 11

Answer: A — Odd numbers: +2 each time → 9.

Q3. 5, 10, 15, 20, ?

  • A) 25
  • B) 30
  • C) 20
  • D) 35

Answer: A — Multiples of 5 → 25.

Q4. 1, 4, 9, 16, ?

  • A) 25
  • B) 24
  • C) 20
  • D) 21

Answer: A — Perfect squares: 5²=25.

Q5. 2, 6, 18, 54, ?

  • A) 162
  • B) 108
  • C) 216
  • D) 180

Answer: A — ×3 each time: 54×3=162.

Q6. 100, 90, 80, 70, ?

  • A) 60
  • B) 50
  • C) 65
  • D) 55

Answer: A — –10 each time: 70–10=60.

Q7. 3, 6, 12, 24, ?

  • A) 48
  • B) 36
  • C) 32
  • D) 44

Answer: A — ×2 each time: 24×2=48.

Q8. 1, 2, 4, 8, 16, ?

  • A) 32
  • B) 28
  • C) 24
  • D) 30

Answer: A — ×2: 16×2=32.

Q9. 10, 20, 30, 40, ?

  • A) 50
  • B) 55
  • C) 45
  • D) 60

Answer: A — Multiples of 10 → 50.

Q10. 0, 1, 3, 6, 10, ?

  • A) 15
  • B) 12
  • C) 14
  • D) 13

Answer: A — Differences: 1,2,3,4,5 → 10+5=15.