RRB JE 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, 6, 12, 20, 30, ?

  • A) 42
  • B) 40
  • C) 44
  • D) 36

Answer: A — Differences: 4,6,8,10,12 → next term = 30+12=42.

Q2. 1, 4, 9, 16, 25, ?

  • A) 36
  • B) 34
  • C) 38
  • D) 30

Answer: A — Perfect squares: 1²,2²,3²,4²,5² → 6²=36.

Q3. 3, 8, 15, 24, 35, ?

  • A) 48
  • B) 50
  • C) 46
  • D) 52

Answer: A — Differences: 5,7,9,11,13 → 35+13=48.

Q4. 5, 10, 20, 40, 80, ?

  • A) 160
  • B) 140
  • C) 120
  • D) 200

Answer: A — Each term ×2: 80×2=160.

Q5. 1, 3, 7, 15, 31, ?

  • A) 63
  • B) 61
  • C) 59
  • D) 65

Answer: A — Each term ×2+1: 31×2+1=63.

Q6. 100, 96, 88, 76, 60, ?

  • A) 40
  • B) 42
  • C) 44
  • D) 38

Answer: A — Differences: –4,–8,–12,–16,–20 → 60–20=40.

Q7. 2, 3, 5, 8, 13, 21, ?

  • A) 34
  • B) 32
  • C) 36
  • D) 30

Answer: A — Fibonacci sequence: each term = sum of two preceding → 13+21=34.

Q8. 7, 14, 28, 56, ?

  • A) 112
  • B) 100
  • C) 108
  • D) 120

Answer: A — ×2 each time: 56×2=112.

Q9. 1, 8, 27, 64, 125, ?

  • A) 216
  • B) 196
  • C) 256
  • D) 169

Answer: A — Cubes: 1³,2³,3³,4³,5³ → 6³=216.

Q10. 144, 121, 100, 81, 64, ?

  • A) 49
  • B) 50
  • C) 48
  • D) 36

Answer: A — Perfect squares descending: 12²,11²,10²,9²,8² → 7²=49.