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?
- (a) 480 km
- (b) 260 km
- (c) 240 km ✓ UPSC's answer
- (d) 120 km
Why the answer is (c)
• The freight train has a 2-hour head start at 40 km/hr, covering 80 km before the express train departs.
• The relative speed between the express train (60 km/hr) and the freight train (40 km/hr) is 20 km/hr.
• The time required for the express train to close the 80 km gap is 80 km / 20 km/hr = 4 hours.
• In these 4 hours, the express train travels 4 hours * 60 km/hr = 240 km from Delhi.
• Therefore, the express train meets the freight train at a distance of 240 km from Delhi.
Why the other options are wrong
- (a) 480 km
- 480 km is the distance the express train would cover in 8 hours, which is double the correct meeting time.
- (b) 260 km
- 260 km is not a standard result of the relative speed calculation for this specific head start and speed difference.
- (d) 120 km
- 120 km is the distance the freight train covers in the first 3 hours, not the meeting point.
Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2017, held on 18 June 2017. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.