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?
- (a) 1 : 3 ✓ UPSC's answer
- (b) 3 : 1
- (c) 2 : 3
- (d) 3 : 2
Why the answer is (a)
• P has 20% copper, Q has 60% copper, and R must have 50% copper (equal copper and zinc).
• By alligation, P : Q = (60 − 50) : (50 − 20) = 10 : 30 = 1 : 3.
• Check: 1 part P and 3 parts Q give (20 + 180)/4 = 50% copper: (a).
Why the other options are wrong
- (b) 3 : 1
- 3 : 1 reverses the ratio and gives only 30% copper.
- (c) 2 : 3
- 2 : 3 gives 44% copper, not 50%.
- (d) 3 : 2
- 3 : 2 gives 36% copper, not 50%.
Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2026, held on 24 May 2026. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.