UPSC Prelims 2017 · CSAT
Question 39: Basic Numeracy
A freight train left Delhi for Mumbai at an average speed of 40 km/hr. Two hours later, an express train left Delhi for Mumbai, following the freight train on a parallel track at an average speed of 60 km/hr. How far from Delhi would the express train meet the freight train?
AnswerUPSC’s answer is (c).
In its 2-hour head start the freight train covers 40 x 2 = 80 km. The express gains 60 - 40 = 20 km every hour, so it needs 80 / 20 = 4 hours to catch up. In that time it runs 60 x 4 = 240 km from Delhi, which equals the freight train's 40 x 6 = 240 km. For chase problems: gap divided by relative speed gives the time, then multiply by one train's speed.
Why each option is right or wrong
- (a) 480 km does not satisfy the catch-up equation.
- (b) 260 km does not satisfy the catch-up equation.
- (c) Correct: 60 x 4 = 240 km, equal to the freight train's 40 x 6.
- (d) 120 km is only the express train's distance in 2 hours.
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.