There are two chemicals which do not react with each other. A container contains 10 litres of the chemical A. One litre of this chemical is removed from it and one litre of the chemical B is poured. Then one litre of the mixture is removed from the container and one litre of B is poured. If this process of replacing one litre of the mixture by one litre of B is performed once more, then what is the volume of B that is present in the container approximately (in percentage)?
Answer & explanation
Answer: (b) 27
Each replacement keeps 9/10 of whatever A is present. After three replacements A = 10 × (0·9)³ = 7·29 litres, so B = 2·71 litres, about 27% of the 10 litres.
- Each step removes 1/10 of the mixture, so A falls by a factor 9/10 each time.
- After 3 steps, A = 10 × 0·9³ = 10 × 0·729 = 7·29 L.
- B = 10 − 7·29 = 2·71 L ≈ 27%.
Remember · Repeated replacement: amount left = original × (1 − removed/total)ⁿ.
Question and answer: UPSC's provisional GS Paper II (2026, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·