Banking Notes / Quantitative Aptitude / Chapter 2
Chapter 2 of 26

Number Series (Missing/Wrong Number)

Spot the pattern fast — the arithmetic, geometric, and mixed series types that dominate bank prelims, with a step-by-step approach to crack any series in under 20 seconds.

📖 ~9 min read 🏦 Banking Quantitative Aptitude

Introduction

A Number Series is a sequence of numbers arranged according to a specific rule. In bank exams you're given a series with one number missing (blank or "?") or one number wrong, and asked to find it. Series questions test pattern recognition speed, not calculation — the faster you spot the logic, the faster you solve.

Common Series Patterns

Pattern TypeRuleExample
Simple Addition/SubtractionConstant difference between terms5, 10, 15, 20, 25 (+5 each)
Multiplication/DivisionConstant ratio between terms3, 6, 12, 24, 48 (×2 each)
Increasing DifferenceDifference itself increases2, 3, 5, 8, 12 (+1, +2, +3, +4)
Square/Cube BasedTerms follow n², n³ patterns1, 4, 9, 16, 25 (n²)
Alternating SeriesTwo interwoven patterns2, 100, 4, 95, 6, 90 (odd pos +2, even pos −5)
Two-Step OperationCombination of two operations per step4, 9, 19, 39, 79 (×2 +1 each)
⚠️ Key Insight: Always check the difference between consecutive terms first. If differences aren't constant, check the differences of differences (second-level pattern), or test ratios and squares/cubes before assuming a random series.

Missing Number vs Wrong Number

In a missing number series, one term is replaced with "?" and you find the pattern from the rest. In a wrong number series, all terms are visible but one breaks the pattern — you identify which term doesn't fit and, if asked, what it should be.

Q. Find the missing number: 7, 15, 31, 63, ?
Check the pattern: 7×2+1=15, 15×2+1=31, 31×2+1=63. So next term = 63×2+1 = 127.
Q. Find the wrong number: 2, 5, 10, 17, 26, 36
Differences: 3, 5, 7, 9 — should continue as 11, giving 26+11=37, not 36. Pattern is n²+1: 1²+1=2, 2²+1=5, 3²+1=10, 4²+1=17, 5²+1=26, 6²+1=37. So 36 is wrong; correct value is 37.

Speed Strategy

  • Scan for a constant difference or ratio first — this covers the majority of prelims-level questions.
  • If the series has 5-6 terms and no obvious pattern, try alternate-term splitting (odd position / even position).
  • For wrong-number questions, don't recompute the whole series — spot-check 2-3 consecutive gaps to isolate the break.
💡 Exam Tip: Number Series is almost always a Prelims-only topic across IBPS/SBI exams and carries easy, quick marks — prioritize it early in the exam to bank time for Data Interpretation later.
✅ Practice Focus: Difference-based and ratio-based series · Square/cube series · Alternating and two-step series · Fast wrong-number spotting.

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