2020 CSAT, all questions
UPSC Prelims 2020 · CSAT
Question 68: Basic Numeracy
A bottle contains 20 litres of liquid A. 4 litres of liquid A is taken out of it and replaced by same quantity of liquid B. Again 4 litres of the mixture is taken out and replaced by same quantity of liquid B. What is the ratio of quantity of liquid A to that of liquid B in the final mixture?
a4 : 1
b5 : 1
c16 : 9
d17 : 8
AnswerUPSC’s answer is (c).
Each replacement leaves a fraction (1 - 4/20) = 4/5 of liquid A behind. After two rounds, A left = 20 × (4/5)² = 20 × 16/25 = 12.8 litres, so B = 20 - 12.8 = 7.2 litres. Ratio A : B = 12.8 : 7.2 = 16 : 9.
Why each option is right or wrong
- (a) 4:1 is the ratio after only one round (16 L : 4 L).
- (b) 5:1 matches no stage.
- (c) Right: 12.8 : 7.2 = 16 : 9.
- (d) 17:8 matches no stage.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2020, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.