Pariksha SaathiWrong Number Series Questions with Solutions
This is Number Series run in reverse — instead of extending a pattern, you’re given a sequence with one term that breaks it, and the job is to find which term doesn’t belong. Here’s the approach, four fully worked examples covering the patterns that come up most, and a few to try yourself.
The approach, quickly
- Check the differences between consecutive terms first. If they form a recognisable pattern themselves (constant, or increasing by a fixed amount, or odd numbers), one term breaking that pattern is your answer.
- If differences don’t reveal a pattern, check the ratio between consecutive terms — a geometric series (each term × a fixed number) is common, and a wrong term will show an inconsistent ratio.
- Check against a known formula family — squares (1, 4, 9, 16, 25, …), n² + a constant, or n(n+1) — since several standard series are built directly from one of these.
- Once you spot the pattern from the terms that DO fit, calculate what the odd-one-out term should have been, rather than just naming which position looks wrong — this confirms your pattern is actually correct.
Worked examples
1. Find the wrong number in the series: 2, 5, 10, 17, 26, 37, 50, 64
Solution
- The differences between consecutive terms are 3, 5, 7, 9, 11, 13, … — each 2 more than the last (this is the n² + 1 series: 1²+1, 2²+1, 3²+1, …).
- Following the pattern, the 8th term should be 8² + 1 = 65, but 64 is given instead.
Answer: 64 (should be 65)
2. Find the wrong number in the series: 3, 6, 12, 24, 48, 90, 192
Solution
- Each term should be double the previous one: 3, 6, 12, 24, 48, 96, 192.
- The 6th term should be 48 × 2 = 96, but 90 is given instead.
Answer: 90 (should be 96)
3. Find the wrong number in the series: 1, 4, 9, 16, 24, 36, 49
Solution
- These are perfect squares: 1², 2², 3², 4², 5², 6², 7² = 1, 4, 9, 16, 25, 36, 49.
- The 5th term should be 5² = 25, but 24 is given instead.
Answer: 24 (should be 25)
4. Find the wrong number in the series: 5, 10, 17, 26, 37, 51, 65
Solution
- The differences are 5, 7, 9, 11, 13, 15 — each 2 more than the last.
- Following the pattern, the 6th term should be 37 + 13 = 50, but 51 is given instead.
Answer: 51 (should be 50)
Try these yourself
Find the pattern from the terms that fit, then check against the answer.
1. Find the wrong number in the series: 4, 9, 16, 25, 35, 49, 64
Answer: 35 (should be 36)
2. Find the wrong number in the series: 2, 6, 12, 20, 32, 42, 56
Answer: 32 (should be 30)
3. Find the wrong number in the series: 7, 14, 28, 55, 112, 224
Answer: 55 (should be 56)
Where this comes up
Wrong Number Series is a regular topic in the Reasoning/General Intelligence and Quantitative Aptitude sections of every exam Pariksha Saathi covers — SSC CGL, SSC MTS, SSC CHSL, IBPS PO, IBPS Clerk, SBI PO and SBI Clerk.
Practice more Wrong Number Series questions
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