A cylindrical overhead tank of radius 2 m and height 7 m is to be filled from an underground tank of size 5.5 m × 4 m × 6 m. How much portion of the underground tank is still filled with water after filling the overhead tank completely?
Answer & explanation
Answer: (a) 1/3
The cylinder holds 22/7 × 2² × 7 = 88 m³, and the full underground tank holds 5.5 × 4 × 6 = 132 m³. After the cylinder is filled, 44 m³ remain — one-third of the underground tank.
- Overhead tank: πr²h = 22/7 × 2 × 2 × 7 = 88 m³.
- Underground tank (taken as full at the start): 5.5 × 4 × 6 = 132 m³.
- Left: 132 − 88 = 44 m³, and 44/132 = 1/3.
Remember · When 7 appears in a radius or height, take π = 22/7; the numbers are usually set to cancel neatly.
Question and answer: UPSC's official GS Paper II (2016, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·