How to Find the Wrong Number in a Series (Bank Exam Method)
To find the wrong number in a series, write the differences (or ratios) between neighbouring terms, spot the pattern that fits most of them, and the one term that breaks it is the wrong number. In 5, 8, 13, 21, 29, 40, 53 the differences should be +3, +5, +7, +9, +11, +13; they are +3, +5, +8, +8, +11, +13, so 21 is wrong (it should be 20).
Key takeaway: One wrong term spoils two neighbouring differences. Look for the pair of differences that are both "off", and the term between them is your answer. Then confirm by rebuilding the whole series with the corrected term.
IBPS Clerk, SBI Clerk and the PO prelims regularly give a set of five number series, and wrong-number sets have become common since the 2023 to 2025 papers. Every series below was generated by a rule in Python, then exactly one term was changed, so each answer is checked. If your exam gives the "missing number" format instead, use our missing number series tricks for bank exams.
4-Step Method for Wrong Number Series Questions
- Look at the growth. Slow growth means addition or subtraction; fast growth (doubling or more) means multiplication; a rise and fall means an alternate pattern.
- Write the first differences (or ratios for fast growth).
- Find the pattern in the differences: constant, rising by a fixed step, squares, cubes, primes, or doubling.
- Locate the two "off" differences. The term shared by them is wrong. Rebuild the series to confirm.
Wrong Number Series Patterns for Bank Exams (Table)
| Pattern | Rule | Example (correct) |
|---|---|---|
| Odd-number differences | +3, +5, +7, ... | 5, 8, 13, 20, 29, 40, 53 |
| Difference of differences | Differences rise by a constant | 7, 13, 23, 37, 55, 77, 103 |
| Square differences | +1², +2², +3², ... | 10, 11, 15, 24, 40, 65, 101 |
| ×a + b | Multiply and add the same numbers | 4, 9, 19, 39, 79, 159, 319 (×2 + 1) |
| ×n (rising multiplier) | ×1, ×2, ×3, ... | 3, 3, 6, 18, 72, 360, 2160 |
| ×n + n | ×1 + 1, ×2 + 2, ... | 2, 3, 8, 27, 112, 565, 3396 |
| n² ± k or n³ ± k | Squares or cubes with a shift | 10, 17, 26, 37, 50, 65, 82 (n² + 1) |
| Alternate | Two rules take turns | 50, 45, 55, 40, 60, 35, 65 |
| Decimal multipliers | ×0.5, ×1, ×1.5, ... | 16, 8, 8, 12, 24, 60, 180 |
Wrong Number Series Questions with Solutions
Find the wrong number in each series.
Q1. 5, 8, 13, 21, 29, 40, 53 Differences should be +3, +5, +7, +9, +11, +13. 13 + 7 = 20, so 21 is wrong.
Q2. 4, 9, 19, 39, 78, 159, 319 Rule ×2 + 1: 39 × 2 + 1 = 79, so 78 is wrong.
Q3. 3, 3, 6, 18, 72, 350, 2160 Rule ×1, ×2, ×3, ×4, ×5, ×6: 72 × 5 = 360, so 350 is wrong (and 360 × 6 = 2160 confirms it).
Q4. 2, 3, 8, 28, 112, 565, 3396 Rule ×1 + 1, ×2 + 2, ×3 + 3, ...: 8 × 3 + 3 = 27, so 28 is wrong (27 × 4 + 4 = 112 confirms).
Q5. 10, 17, 26, 37, 50, 64, 82 Rule n² + 1 for n = 3 to 9: 8² + 1 = 65, so 64 is wrong.
Q6. 10, 11, 15, 24, 40, 64, 101 Differences should be 1, 4, 9, 16, 25, 36 (squares). 40 + 25 = 65, so 64 is wrong. Notice that the same number can be wrong for different reasons; always find the rule.
Q7. 16, 8, 8, 12, 24, 60, 190 Rule ×0.5, ×1, ×1.5, ×2, ×2.5, ×3: 60 × 3 = 180, so 190 is wrong.
Q8. 50, 45, 55, 40, 60, 30, 65 Rule −5, +10, −15, +20, −25, +30: 60 − 25 = 35, so 30 is wrong (35 + 30 = 65 confirms).
Q9. 7, 13, 23, 37, 55, 76, 103 Differences 6, 10, 14, 18, 22, 26 (rising by 4). 55 + 22 = 77, so 76 is wrong.
Q10. 7, 26, 63, 124, 217, 342, 511 Rule n³ − 1 for n = 2 to 8: 6³ − 1 = 215, so 217 is wrong.
Wrong Number Series Short Tricks
- One wrong term breaks two differences. In Q1 the differences around 21 are +8 and +8 instead of +7 and +9; the sum (+16) is still correct, which is a strong hint.
- Check the last term when the series grows fast: it often confirms the rule (Q3, Q7).
- Learn squares to 30 and cubes to 15. Many series sit just above or below them (Q5, Q10).
- If differences jump around, try alternate terms (Q8) or a second-level difference (Q9).
- Decimal multipliers (×0.5, ×1.5) appear when the series first falls and then rises (Q7).
- Primes sometimes hide in the terms or the differences: 2, 3, 5, 7, 11, 13, 17, 19, 23.
Speed with squares, cubes and quick multiplication comes from the same habits as approximation questions for bank exams and the Vedic maths shortcuts.
More Wrong Number Series Questions (Mixed Patterns)
Q11. 6, 7, 16, 52, 208, 1045, 6276 Rule ×1 + 1, ×2 + 2, ×3 + 3, ×4 + 4, ...: 16 × 3 + 3 = 51, so 52 is wrong (51 × 4 + 4 = 208 confirms).
Q12. 2, 15, 4, 12, 8, 10, 16, 6 Two series take turns. Odd places: 2, 4, 8, 16 (×2). Even places: 15, 12, 9, 6 (−3). So 10 is wrong; it should be 9.
Q13. 120, 99, 80, 63, 48, 36, 24 Rule n² − 1 going down from n = 11: 120, 99, 80, 63, 48, 35, 24. So 36 is wrong.
Q14. 1, 2, 6, 21, 89, 445, 2676 Rule ×1 + 1, ×2 + 2, ×3 + 3, ...: 21 × 4 + 4 = 88, so 89 is wrong (88 × 5 + 5 = 445 confirms).
Time Plan for a Number Series Set in Prelims
A set of five series should take about four minutes. Spend the first 20 seconds on each series only on the growth check (slow, fast, up and down). If the first-difference row shows nothing in 30 seconds, write the ratio row instead, and if that fails, skip and return. Most candidates lose time by trying second differences on a series that is really ×a + b. Because these questions carry the same marks as easier ones, never spend more than a minute on one.
Wrong Number Series Quick Revision Table
| What you see | Try first |
|---|---|
| Slow, steady rise | First differences |
| Differences themselves rise steadily | Second differences |
| Terms near 10, 17, 26, 37 or 8, 26, 63 | n² ± k or n³ ± k |
| Terms roughly double or triple | ×a + b |
| Growth speeds up each step | ×1, ×2, ×3 ... (with or without + n) |
| Rise and fall | Alternate terms or alternating signs |
| Fall then rise | Decimal multipliers (×0.5, ×1, ×1.5) |
| Eight terms | Two interleaved series |
Missing Number Series vs Wrong Number Series
| Point | Missing number | Wrong number |
|---|---|---|
| What you find | The term marked "?" | The term that breaks the rule |
| Number of terms shown | Usually 6 with one blank | Usually 7, all shown |
| Key check | Rule must produce the next term | Rule must fit all terms but one |
| Typical trap | Two rules fit the given terms | Changing the wrong term "fixes" one difference but breaks another |
Common Mistakes in Wrong Number Series
- Picking the first term that looks odd without rebuilding the series.
- Stopping at first differences when the pattern is in second differences (Q9).
- Missing alternate patterns because the series rises and falls (Q8).
- Ignoring that 64 can be wrong as "n² + 1" in one series and as "square differences" in another (Q5, Q6).
Wrong Number Series Practice Questions (with Answers)
- 2, 5, 14, 41, 122, 366, 1094. Answer: 366 (×3 − 1; 122 × 3 − 1 = 365).
- 3200, 1600, 800, 400, 200, 110, 50. Answer: 110 (÷2; should be 100).
- 3, 8, 15, 25, 35, 48, 63. Answer: 25 (n² − 1; should be 24).
- 11, 18, 25, 32, 40, 46, 53. Answer: 40 (+7; should be 39).
- 3, 4, 6, 10, 18, 36, 66. Answer: 36 (×2 − 2; should be 34).
- 2, 3, 5, 7, 11, 15, 17. Answer: 15 (primes; should be 13).
- 5, 6, 8, 12, 21, 36, 68. Answer: 21 (differences 1, 2, 4, 8, 16, 32; should be 20).
- 8, 12, 18, 28, 40.5, 60.75. Answer: 28 (×1.5; should be 27).
- 50, 45, 55, 40, 60, 35, 70. Answer: 70 (−5, +10, −15, +20, −25, +30; should be 65).
- 4, 9, 19, 39, 79, 160, 319. Answer: 160 (×2 + 1; should be 159).
Practise full sets in the SBI PO Quantitative Aptitude Number Series Mock Test and the SSC CGL Number Series Mock Test 01.
Wrong Number Series FAQs
How do you find the wrong number in a series quickly?
Write the differences or ratios, find the rule that fits most of them, and pick the term that sits between the two differences that break the rule. Then rebuild the series to confirm.
How many number series questions come in bank exams?
IBPS and SBI clerk and PO prelims usually have a set of about five number series questions, either missing-number or wrong-number.
What are the most common wrong number series patterns?
Odd-number or square differences, ×a + b, rising multipliers (×1, ×2, ×3), n² or n³ with a shift, and alternate patterns.
Is wrong number series the same as odd one out?
They are closely related. Some PO papers call it "odd one out" or "find the wrong term"; the method is the same.
What if two different terms seem wrong?
Correct each one in turn and rebuild the series. Only one correction makes every difference fit the rule.
Practice Next
- Mock tests: SSC CGL Number Series Mock Test 02 and IBPS Clerk Numerical Ability Sectional Mock Test 05.
- New topic-wise test: IBPS Clerk Number Series Mock Test 01: Missing and Wrong Number Series (15 Questions).
- SBI number series tests: SBI PO Number Series Mock Test 02 and SBI Clerk Number Series Mock Test 02.
- Related notes: missing number series tricks for bank exams, quadratic equation tricks for bank exams, probability questions for bank exams, unit digit tricks for SSC CGL and alphanumeric series tricks for bank exams.
- Exam details: SBI Clerk 2026 notification.
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