2015 CSAT, all questions
UPSC Prelims 2015 · CSAT
Question 71: Basic Numeracy
Two cities A and B are 360 km apart. A car goes from A to B with a speed of 40 km/hr and returns to A with a speed of 60 km/hr. What is the average speed of the car ?
a45 km/hr
b48 km/hr
c50 km/hr
d55 km/hr
AnswerUPSC’s answer is (b).
Average speed over a round trip is total distance divided by total time, not the mean of the two speeds. A to B takes 360 ÷ 40 = 9 hours; B to A takes 360 ÷ 60 = 6 hours. Total distance 720 km in 15 hours gives 48 km/h. The shortcut for equal distances is 2 × 40 × 60 ÷ (40 + 60) = 48.
Why each option is right or wrong
- (a) Wrong: 45 km/h would take 16 hours for 720 km, but the trip takes 15.
- (b) Right: 720 ÷ 15 = 48 km/h.
- (c) Wrong: 50 is the simple average of 40 and 60, which ignores that more time is spent at the slower speed.
- (d) Wrong: 55 is higher than even the simple average of the two speeds.
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.