Replace the incorrect term by the correct term in the given sequence
3, 2, 7, 4, 13, 10, 21, 18, 31, 28, 43, 40
where odd terms and even terms follow the same pattern.
Answer & explanation
Answer: (a) 0
The odd-place terms rise by 4, 6, 8, 10, 12. The even-place terms follow the same steps only from 4 onwards, so their first term should be 4 − 4 = 0, not 2.
- Odd places: 3, 7, 13, 21, 31, 43 — differences 4, 6, 8, 10, 12.
- Even places: 2, 4, 10, 18, 28, 40 — differences 2, 6, 8, 10, 12.
- Only the first even-place difference breaks the pattern; to match, the first even term must be 4 − 4 = 0.
- So the incorrect term 2 should be replaced by 0.
- Check: each even-place term is then 3 less than the odd-place term before it — 0, 4, 10, 18, 28, 40.
Remember · In an alternating series, split it into two sub-series and compare their differences; the misfit shows itself.
Question and answer: UPSC's official GS Paper II (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·