NDA NA 2026 — General Intelligence & Reasoning: Series Completion Concept Guide + 10 MCQs

Master Series Completion for NDA NA 2026. This guide covers theory, shortcuts, and 10 practice MCQs.

Concept: Series Completion

Series Completion finds the next number by identifying the mathematical pattern.

Common Patterns

  • Arithmetic: constant difference
  • Geometric: constant ratio
  • Squares: 1,4,9,16...
  • Cubes: 1,8,27,64...
  • Fibonacci: each = sum of two before
  • Primes: 2,3,5,7,11...
  • Factorials: 1,2,6,24,120...

⚡ Shortcuts & Tricks

  • Find differences between terms first
  • If irregular, find differences of differences
  • Check if they are squares or cubes
  • Large ratio differences = geometric series

Practice MCQs (10 Questions)

Q1. 2, 4, 8, 16, 32, ?

  • A. 64
  • B. 48
  • C. 56
  • D. 72
Answer: A — Geometric series: multiply by 2 each time. 32×2=64.

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

  • A. 36
  • B. 30
  • C. 32
  • D. 34
Answer: A — Perfect squares: 1², 2², 3², 4², 5², 6²=36.

Q3. 1, 1, 2, 3, 5, 8, ?

  • A. 13
  • B. 11
  • C. 12
  • D. 14
Answer: A — Fibonacci: 5+8=13.

Q4. 3, 9, 27, 81, ?

  • A. 243
  • B. 162
  • C. 216
  • D. 189
Answer: A — Powers of 3: 3⁵=243.

Q5. 1, 8, 27, 64, ?

  • A. 125
  • B. 100
  • C. 112
  • D. 144
Answer: A — Cubes: 5³=125.

Q6. 11, 13, 17, 19, 23, ?

  • A. 29
  • B. 27
  • C. 25
  • D. 31
Answer: A — Prime numbers in sequence. Next prime after 23 is 29.

Q7. 100, 50, 25, 12.5, ?

  • A. 6.25
  • B. 5
  • C. 8
  • D. 10
Answer: A — Each term halved: 12.5÷2=6.25.

Q8. 1, 2, 6, 24, 120, ?

  • A. 720
  • B. 600
  • C. 360
  • D. 480
Answer: A — Factorial: 6!=720.

Q9. 2, 5, 10, 17, 26, ?

  • A. 37
  • B. 35
  • C. 33
  • D. 39
Answer: A — n²+1: 6²+1=37.

Q10. 144, 121, 100, 81, ?

  • A. 64
  • B. 72
  • C. 60
  • D. 68
Answer: A — Descending perfect squares: 8²=64.

Summary

Always find differences first. Then check squares, cubes, primes, or Fibonacci patterns.