A daily train is to be introduced between station A and station B starting from each end at 6 AM and the journey is to be completed in 42 hours. What is the number of trains needed in order to maintain the shuttle service ?
- (a) 2
- (b) 3
- (c) 4 ✓ UPSC's answer
- (d) 7
Why the answer is (c)
• A train leaving A at 6 AM reaches B 42 hours later, at midnight on the second night, and can take the next 6 AM departure from B, 48 hours after it started.
• It is back at A after another 48 hours, so each train completes a round trip in 96 hours = 4 days.
• The service needs one departure from each end every day, i.e. 2 departures a day, while each train provides 2 departures in 4 days.
• So 2 × 4 ÷ 2 = 4 trains are needed.
• Hence 4 trains, option (c).
Why the other options are wrong
- (a) 2
- Two trains cover only half the daily departures in each 4-day cycle.
- (b) 3
- Three trains cannot supply two departures every day on a 4-day round trip.
- (d) 7
- Seven trains are more than the four needed.
Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2016, held on 7 August 2016. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.