Take this series: 4, 9, 19, 40, 79, 159. Most people stare at it, try +5, +10, +21, give up, and move on. One number in it is wrong, and once you know where to look, you can find it in about 15 seconds.
That is the whole idea behind wrong number series questions, and the missing number series type works on the same logic. In this post I explain the method, show 14 solved questions, and give you a short practice set. Download link for the free PDF is at the end.
“I used to identify a pattern based on the first two terms, but it would turn out to be incorrect.”
Table of Contents
Wrong Number Series: How to Find the Odd One Out
In a wrong number series, every term follows one rule except one. Your job is to find the term that breaks the rule. The common mistake is to build the rule from the first two terms, and that is risky, because the wrong number might be one of them.
The method I follow
- Look at the growth. Slow growth points to differences. Fast growth points to multipliers, squares or cubes.
- Find the rule from the terms in the middle of the series, where two or three consecutive terms look consistent.
- Run the rule forward and backward. The first term that does not fit is your wrong number.
- Check the last term. If the end of the series still fits the rule, you have found the right pattern.
Solved examples (wrong number series)
1. 4, 9, 19, 40, 79, 159
The growth is fast, so test ×2 + 1: 4 → 9 → 19 → 39 → 79 → 159. The fourth term should be 39, not 40.
Wrong number: 40
2. 5, 7, 11, 17, 25, 36, 47
The differences should be 2, 4, 6, 8, 10, 12. The actual differences are 2, 4, 6, 8, 11, 11. Since 25 + 10 = 35 and 35 + 12 = 47, the term 36 breaks the pattern.
Wrong number: 36 (should be 35)
3. 3, 6, 11, 18, 27, 37, 51
These are n² + 2: 3, 6, 11, 18, 27, 38, 51 (for n = 1 to 7). The sixth term should be 38.
Wrong number: 37
4. 2, 4, 12, 48, 250, 1440
The multipliers should be ×2, ×3, ×4, ×5, ×6. From the end, 1440 ÷ 6 = 240, and 48 × 5 = 240.
Wrong number: 250 (should be 240)
5. 2, 3, 8, 28, 112, 565
Pattern: ×1 + 1, ×2 + 2, ×3 + 3, ×4 + 4, ×5 + 5. Check: 8 × 3 + 3 = 27, 27 × 4 + 4 = 112, 112 × 5 + 5 = 565.
Wrong number: 28 (should be 27)
When I see growth, I check the ratios first; the differences come later.
Missing Number Series Tricks: How to Find the Missing Number
In a missing number series, the series is correct and one position has a question mark. The question mark can be anywhere: in the middle, at the start, or at the end. If it is in the middle, build the rule from the terms on both sides.
Tip: do not only look at the terms before the gap. The term after the gap often confirms the rule.
Solved examples (missing number series)
6. 5, 11, 23, 47, ?, 191
Pattern: ×2 + 1. Check: 5 × 2 + 1 = 11, 11 × 2 + 1 = 23, 23 × 2 + 1 = 47, 47 × 2 + 1 = 95, 95 × 2 + 1 = 191.
Missing number: 95
7. 2, 5, 11, ?, 32, 47
Differences: 3, 6, 9, 12, 15. So 11 + 9 = 20, then 20 + 12 = 32, then 32 + 15 = 47.
Missing number: 20
8. ?, 12, 36, 108, 324
The multiplier is ×3. Working backward, 12 ÷ 3 = 4.
Missing number: 4
9. 1, 2, 6, 24, ?, 720
Multipliers: ×2, ×3, ×4, ×5, ×6. So 24 × 5 = 120, and 120 × 6 = 720.
Missing number: 120
Wrong vs Missing: Quick Comparison
| Wrong number series | Missing number series | |
|---|---|---|
| What is given | Full series with one incorrect term | Series with one blank |
| What you find | The incorrect term | The blank value |
| Where to start | The middle terms | Terms on both sides of the blank |
| Quick check | The rule must fit every other term | The rule must fit the term after the blank |
Practice Set
Try these without looking at the explanation. Aim for 40 to 50 seconds each.
Wrong number series
- 6, 13, 27, 55, 112, 223
- 10, 12, 16, 23, 30, 40
- 2, 6, 24, 120, 725, 5040
- 8, 27, 64, 125, 215, 343
Missing number series
5. 3, 7, 15, 31, ?, 127
6. 4, 6, 12, 30, ?, 315
7. 7, 9, 14, 23, ?, 57
8. 2, 5, 10, 17, ?, 37
Answers
- 112 (should be 111). Pattern ×2 + 1: 6, 13, 27, 55, 111, 223.
- 23 (should be 22). Differences 2, 4, 6, 8, 10: 10, 12, 16, 22, 30, 40.
- 725 (should be 720). Multipliers ×3, ×4, ×5, ×6, ×7: 2, 6, 24, 120, 720, 5040.
- 215 (should be 216). These are cubes of 2 to 7: 8, 27, 64, 125, 216, 343.
- 63. Pattern ×2 + 1: 31 × 2 + 1 = 63, and 63 × 2 + 1 = 127.
- 90. Multipliers ×1.5, ×2, ×2.5, ×3, ×3.5: 30 × 3 = 90, and 90 × 3.5 = 315.
- 37. Differences 2, 5, 9, 14, 20 (the gaps grow by 3, 4, 5, 6): 23 + 14 = 37, and 37 + 20 = 57.
- 26. Pattern n² + 1: 2, 5, 10, 17, 26, 37.
Where I Lose Marks in Number Series
- I built the rule from the first two terms and stuck to it, even when the third term did not fit.
My short checklist before moving on to the next question:
- Does my rule fit every term, not just the first three?
- If the series zigzags, did I split it into odd and even positions?
- Did I double-check the arithmetic of the last step?
Earlier, a number series question used to take me 10–14 seconds; now, it takes 4–6 seconds.
Patterns That Appear Most in Number Series Questions
Both types are built from the same small set of ideas, so learn these first:
- Differences: constant, increasing (+2, +4, +6) or prime-number differences
- Multipliers: constant (×2) or increasing (×2, ×3, ×4)
- Multiply and add: ×2 + 1, or ×n + n
- Squares and cubes: n² + 1, n³ − 1
- Alternating series: odd and even positions follow different rules
For the full list with examples, read our Number Series for Bank Exams: 12 Patterns post. Revising the Squares and Cubes Chart first makes the square and cube patterns much faster.
Quick FAQs
What is the difference between missing and wrong number series?
In a missing series, one term is blank and you fill it. In a wrong series, all terms are given, but one does not fit the rule and you identify it.
Should I build the rule from the first two terms?
Not in a wrong number series, because the wrong term can be one of the first two. Build the rule from the middle terms, where consecutive terms agree with each other.
Are these questions asked in IBPS PO and SBI PO?
Number series is regularly practiced for bank exams, but the exact pattern changes from year to year. Check the latest exam notification and the previous year’s paper to see how it is being asked now.
Download the Practice Set PDF
Get the missing and wrong number series practice set PDF with 9 solved examples and the 8-question practice set
👉 [Download Missing and Wrong Number Series PDF] (join our Telegram community to get the link)
If you are new to patterns, read our Number Series for Bank Exams: 12 Patterns post first, and revise the Squares and Cubes Chart before you practice. Patterns with squares and cubes become much faster.
Next post: SBI PO Mains Data Interpretation: 15 Questions with Step-by-Step Solutions
