UPSC Prelims 1995 · General Studies
Question 136: Basic Numeracy
A person travelled from one place to another at an average speed of 40 kilometres/hour and back to the original place at an average speed of 50 kilometres/hour. What is his average speed in kilometres/hour during the entire roundtrip?
AnswerThe answer is (c).
Average speed over a round trip is total distance divided by total time, not the mean of the two speeds. For a distance d each way, time is d/40 + d/50 = 9d/200, so average speed = 2d / (9d/200) = 400/9 km/h, about 44.4. The distance cancels, so it need not be known. This is the harmonic mean of 40 and 50.
Why each option is right or wrong
- (a) 45 is the plain average, which is wrong since more time is spent at the slower speed.
- (b) 20 is too low.
- (c) Right: 400/9 km/h.
- (d) The distance cancels out, so it is not needed.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 1995, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.