What is X in the sequence 1, 3, 6, 11, 18, X, 42?
Answer & explanation
Answer: (c) 29
The gaps between terms are consecutive prime numbers: 2, 3, 5, 7, 11, 13. So X = 18 + 11 = 29, and 29 + 13 = 42 confirms it.
- Differences: 3 − 1 = 2, 6 − 3 = 3, 11 − 6 = 5, 18 − 11 = 7 — consecutive primes.
- The next primes are 11 and 13, so X = 18 + 11 = 29.
- Check: 29 + 13 = 42, the last term.
Remember · Try differences first; gaps of 2, 3, 5, 7 signal a prime-number pattern.
Question and answer: UPSC's official GS Paper II (2025, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·