UPSC Prelims 2026 · CSAT Paper 2 · Set DMathematicsQ 59 / 80

UPSC Prelims 2026 CSAT Paper 2 — Question 59

Question

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
(c)29
(d)31

UnlockIAS answer: (b)27

Solution

After three replacements, A remaining = 10 × (9/10)³ = 7.29 L, so B = 2.71 L, i.e. about 27%.

UnlockIAS expert key, prepared 24 May 2026 and cross-verified by re-deriving the answer — this is our key, not the official UPSC provisional key (awaited).

Section

Mathematics

Topic Reference

Numeracy — Mixture & replacement

Official UPSC key still awaited

UPSC has announced (PIB Release 2262441, 18 May 2026) that the provisional answer key for CSE Prelims 2026 will be released “soon after Exam”. Until then, use the CSAT score calculator for your qualifying-status estimate, and join the per-question discussion below.

CSAT Score Calculator

Check your 33% qualifying status

Official Notification

PIB Release 2262441

Back to all 80 CSAT questions