UPSC Prelims 2026 · CSAT
Question 22: Basic Numeracy
What is the minimum number of times one needs to measure to get 298 litres of water from a tank, if the measuring cylinders have capacities 1 litre, 6 litres, 25 litres and 100 litres?
AnswerUPSC’s answer is (b).
298 = 3 × 100 − 2 × 1. Measure out 300 litres with the 100-litre cylinder three times, then put back 2 litres with the 1-litre cylinder twice: 5 measures. No four-measure combination of 1, 6, 25 and 100 (adding or returning) reaches 298. If only additions were allowed, the minimum would be 12 (2 × 100 + 2 × 25 + 8 × 6), which is not an option, so the question intends measures that can be put back.
Why each option is right or wrong
- (a) Wrong: no four measures of 1, 6, 25, 100 give 298.
- (b) Right: 3 × 100 − 2 × 1 uses 5 measures.
- (c) Wrong: more than the minimum of 5.
- (d) Wrong: 13 comes from a wasteful additive route (2 × 100 + 3 × 25 + 3 × 6 + 5 × 1), not the minimum.
Only the provisional key (27 May 2026) was found; the final key replaces it when found.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2026, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.