UPSC Prelims 2026 · CSAT
Question 65: Basic Numeracy
An alloy P contains 20% copper and 80% zinc by weight. Another alloy Q contains 60% copper and 40% zinc by weight. A third alloy R is to be prepared from P and Q so that it contains equal amount of copper and zinc. In what ratio, amounts of P and Q be mixed in order to get R?
AnswerUPSC’s answer is (a).
R must be 50% copper. Mix P and Q in the ratio p : q. Copper: 20p + 60q = 50(p + q), so 10q = 30p and q = 3p. Hence P : Q = 1 : 3. Alligation check: P is 30 below the target of 50 and Q is 10 above it, so the amounts are in the inverse ratio, P to Q = 10 : 30 = 1 : 3.
Why each option is right or wrong
- (a) Right: P : Q = 1 : 3 gives exactly 50% copper.
- (b) Wrong: 3 : 1 is the reverse and gives 30% copper.
- (c) Wrong: 2 : 3 gives 44% copper.
- (d) Wrong: 3 : 2 gives 36% copper.
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.