A train crosses a bridge — Aptitude Question
A train crosses a bridge: A train 150 m long running at 72 km/h crosses a bridge 250 m long. How long does it take? Answer and worked solution on CodeKairo.
- Topic: Time, Speed & Distance
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 60 seconds
Question
A train 150 m long running at 72 km/h crosses a bridge 250 m long. How long does it take?
Options
- 15 s
- 18 s
- 20 s
- 24 s
Show the answer and the worked solution
Correct answer: C. 20 s
Worked solution
- Distance = 150 + 250 = 400 m.
- Speed = 72 × 5/18 = 20 m/s.
- Time = 400 ÷ 20 = 20 s.
Answer: 20 s.
The idea
Convert the speed to m/s first so that every quantity is in metres and seconds. Then it is one division.
Shortcut. 72 km/h is the stock 20 m/s (36 km/h = 10 m/s, doubled), so the 400 m of train plus bridge take 400 ÷ 20 = 20 s.
Common trap. Using only the bridge's 250 m gives 250 ÷ 20 = 12.5 s, and 15 s is the nearest option; the train has crossed only when its tail leaves the bridge, so it covers 150 + 250 = 400 m at 20 m/s.