In the sequence 1, 5, 7, 3, 5, 7, 4, 3, 5, 7, how many such 5s are there which are not immediately preceded by 3 but are immediately followed by 7?
Answer & explanation
Answer: (a) 1
The sequence has three 5s. The first is preceded by 1 and followed by 7; the other two are both preceded by 3. Only one 5 meets both conditions.
- 5s are at positions 2, 5 and 9: (1, 5, 7), (3, 5, 7), (3, 5, 7).
- Position 2: preceded by 1, followed by 7 → counts.
- Positions 5 and 9: preceded by 3 → excluded.
- Count = 1.
Remember · Mark every target symbol first, then test both neighbours of each; don't count by eye.
Question and answer: UPSC's official GS Paper II (2019, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·