2015 CSAT, all questions
UPSC Prelims 2015 · CSAT
Question 53: Basic Numeracy
Two pipes A and B can independently fill a tank completely in 20 and 30 minutes respectively. If both the pipes are opened simultaneously, how much time will they take to fill the tank completely ?
a10 minutes
b12 minutes
c15 minutes
d25 minutes
AnswerUPSC’s answer is (b).
Add the rates. A fills 1/20 of the tank per minute and B 1/30. Together they fill 1/20 + 1/30 = 3/60 + 2/60 = 5/60 = 1/12 per minute. So the tank fills in 12 minutes. With both open the time must be less than the faster pipe's 20 minutes alone, which rules out 25 at once.
Why each option is right or wrong
- (a) Wrong: 10 minutes would need a combined rate of 1/10.
- (b) Right: the combined rate is 1/12 per minute.
- (c) Wrong: 15 minutes would need a combined rate of 1/15.
- (d) Wrong: 25 minutes is longer than pipe A alone takes (20), impossible with both open.
Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2015, General Studies Paper II (CSAT), Series A. The paper on upsc.gov.in.