2001 General Studies, all questions
UPSC Prelims 2001 · General Studies
Question 150: Basic Numeracy
A person travels from X to Y at a speed of 40 kmph and returns by increasing his speed by 50%. What is his average speed for both the trips?
a36 kmph
b45 kmph
c48 kmph
d50 kmph
AnswerThe answer is (c).
The return speed is 40 x 1.5 = 60 km/h. For equal distances d each way, average speed = total distance / total time = 2d / (d/40 + d/60) = 2 x 40 x 60 / (40 + 60) = 48 km/h. The simple mean of 50 is wrong because more time is spent at the slower speed. Remember that the harmonic mean applies for equal distances.
Why each option is right or wrong
- (a) 36 km/h is below even the slower speed of 40 km/h, so it cannot be an average of the two trips.
- (b) 45 km/h is not the correct average for the two trips.
- (c) Right: 48 km/h.
- (d) 50 km/h is the simple average, which ignores the time spent at each speed.
Explanation: written by UPSC Grade, not by UPSC.
Source: UPSC, Civil Services (Preliminary) Examination 2001, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.