UPSC Prelims 2017 · CSAT
Question 68: Basic Numeracy
The average rainfall in a city for the first four days was recorded to be 0·40 inch. The rainfall on the last two days was in the ratio of 4 : 3. The average of six days was 0·50 inch. What was the rainfall on the fifth day?
AnswerUPSC’s answer is (c).
The first four days total 4 x 0.40 = 1.6 inches. Six days at an average of 0.50 total 6 x 0.50 = 3.0 inches. So the last two days had 3.0 - 1.6 = 1.4 inches together. Dividing 1.4 in the ratio 4 : 3, the fifth day gets 4/7 x 1.4 = 0.8 inch and the sixth gets 0.6. Convert averages to totals first.
Why each option is right or wrong
- (a) 0.6 inch is the sixth day's rainfall.
- (b) 0.7 inch would split 1.4 equally, a ratio of 1 : 1.
- (c) Correct: 4/7 x 1.4 = 0.8 inch.
- (d) 0.9 inch would leave 0.5 for the sixth day, a ratio of 9 : 5, not 4 : 3.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2017, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.