UPSC CSE 2026 · PYQ · Mixtures and Alligations / Successive Replacement · medium
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)?
A.25
B.27✓ Correct
C.29
D.31
Explanation
Using the replacement formula, after n replacements of 1 litre out of 10 litres, the fraction of original chemical A remaining = (1 − 1/10)^n = (9/10)^n. Here n = 3, so A remaining = (9/10)^3 = 729/1000 = 72.9% of the container. Since the container always holds 10 litres total, the percentage of B in the container = 100 − 72.9 = 27.1%, approximately 27%.
💡 Practice unlimited UPSC CSE PYQs + AI-tracked progress on each topic. Sign up free →
Want more UPSC CSE practice?
Free daily 10-Q quiz · adaptive mocks · 4,000+ verified PYQs · AI doubt solver in Hindi + English