Wrong-Number and Missing-Term Strategy

Step 1: Glance at growth speed — slow/linear suggests addition; fast suggests multiplication. Step 2: Compute the first 2-3 differences or ratios. Step 3: Establish the rule from the EARLY terms (they are most often correct). Step 4: Apply the rule forward and the first term that violates it is the answer. Speed tip: never assume the wrong term is in the middle — scan systematically from the start. If the rule holds for the first three terms, the break point is your answer. Always double-check by confirming the rule resumes correctly AFTER replacing the wrong term, which validates your choice.

For missing-term questions (a blank in the middle), use BOTH neighbours. Find the rule from terms before AND after the gap, then confirm both directions meet at the blank. Example: 4, 8, ?, 32, 64 is doubling, so the blank is 16 (8×2=16 and 16×2=32). Memory aid: if you can verify the rule on either side of the gap, your answer is almost certainly correct. For alternating series with a blank, identify which sub-pattern the blank belongs to (odd or even position) and solve only that strand. This avoids confusion and saves time on Clerk's speed-sensitive section.

Trap 1: A single arithmetic 'fits-twice' coincidence — always verify a rule across at least three terms, not two. Trap 2: Confusing ×2 with +n when early small numbers behave similarly (2,4 could be ×2 or +2). Use the third term to disambiguate. Trap 3: Mixed series where you stop at the first rule that 'almost' works. For IBPS Clerk, allocate roughly 30-40 seconds per series; if a pattern doesn't emerge in two passes, mark it and move on. Summary: confirm rules on multiple terms, watch for ×/+ ambiguity, and never burn 2+ minutes on a single number-series question.

Cover the answer, recall, then check. 12 cards on the wrong-number and missing-term series strategy for IBPS Clerk Prelims.

Q1. In a "find the wrong number" series, how many terms follow the true rule?
A1. All but one. Establish the rule from the clean terms, then find the single term that breaks it.

Q2. Series: 3, 5, 9, 17, 33, 66, 129. Which number is wrong?
A2. 66. Rule is ×2 − 1: 3, 5, 9, 17, 33, 65, 129. So 66 should be 65.

Q3. Best way to lock the rule before hunting the odd term?
A3. Use the first 2–3 gaps or ratios; they are usually correct, so build the pattern forward from the start.

Q4. Series: 2, 3, 6, 15, 45, 157.5, 630 — pattern? (missing/verify)
A4. Multipliers ×1.5, ×2, ×2.5, ×3, ×3.5, ×4. All terms fit, so none is wrong.

Q5. Missing term: 6, 11, 21, 36, ?, 81. Find it.
A5. 56. Differences 5, 10, 15, 20, 25 → 36 + 20 = 56 (and 56 + 25 = 81 ✓).

Q6. Why check the pattern from both ends on a missing-term item?
A6. It confirms the gap you insert keeps the sequence consistent on the left and the right of the blank.

Q7. Series: 4, 6, 10, 18, 34, 64, 130. Find the wrong term.
A7. 64. Rule ×2 − 2: 4, 6, 10, 18, 34, 66, 130. So 64 should be 66.

Q8. If two terms both seem to "break" the rule, what likely happened?
A8. You picked the wrong rule. Re-derive it from the earliest clean terms — only one term is actually wrong.

Q9. Series: 5, 6, 14, 45, 184, 925 — verify the rule.
A9. ×1+1, ×2+2, ×3+3, ×4+4, ×5+5: 5→6→14→45→184→925. All correct.

Q10. Missing term: 7, 14, 28, ?, 112. Find it.
A10. 56. Constant ratio ×2 → 28 × 2 = 56 (and 56 × 2 = 112 ✓).

Q11. Quick sanity rule for wrong-number problems?
A11. Replace the suspect term with the value your rule predicts; if the rest of the chain then fits perfectly, you found it.

Q12. Common trap in wrong-number series?
A12. Assuming the first term is wrong. The starting terms almost always set the correct rule — suspect the later ones.

Worked Example

Problem: Find the wrong number in the series: 2, 6, 12, 21, 30, 42, 56.

Solution:
First guess the intended rule from the differences.
6 − 2 = 4
12 − 6 = 6
21 − 12 = 9
30 − 21 = 9
42 − 30 = 12
56 − 42 = 14
The differences 4, 6, ?, ?, 12, 14 suggest the pattern of even numbers 4, 6, 8, 10, 12, 14. Then the correct terms should be 2, 6, 12, 20, 30, 42, 56 (each term = n² + n). The value 21 breaks this; it should be 20.

Answer: The wrong number is 21; it should be 20.