What is X in the sequence 24, X, 12, 18, 36, 90?
Answer & explanation
Answer: (b) 12
Each term is the previous one multiplied by 0.5, 1, 1.5, 2 and 2.5 in turn. So X = 24 × 0.5 = 12, and 12 × 1 gives the next term, 12.
- From 12 onwards: 12 × 1.5 = 18, 18 × 2 = 36, 36 × 2.5 = 90. The multiplier rises by 0.5 each time.
- Going backwards, the earlier multipliers are 1 and 0.5.
- So X = 24 × 0.5 = 12, and X × 1 = 12 matches the third term.
- Check: 24 × 0.5 = 12, 12 × 1 = 12, 12 × 1.5 = 18, 18 × 2 = 36, 36 × 2.5 = 90.
Remember · If differences do not work, try ratios; ratios rising by a fixed step (0.5, 1, 1.5 …) are a common UPSC 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). ·