Two trains passing each other — Aptitude Question
Two trains passing each other: Two trains 140 m and 160 m long run towards each other at 60 km/h and 48 km/h. Answer and worked solution on CodeKairo.
- Topic: Time, Speed & Distance
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 75 seconds
Question
Two trains 140 m and 160 m long run towards each other at 60 km/h and 48 km/h. How long do they take to cross each other?
Options
- 8 s
- 10 s
- 12 s
- 15 s
Show the answer and the worked solution
Correct answer: B. 10 s
Worked solution
- Relative speed = 60 + 48 = 108 km/h = 108 × 5/18 = 30 m/s.
- Distance = 140 + 160 = 300 m.
- Time = 300 ÷ 30 = 10 s.
Answer: 10 s.
The idea
Opposite directions add speeds, same direction subtracts them. The distance to clear is always the sum of both lengths.
Shortcut. 108 km/h is 3 × 36 km/h, and 36 km/h = 10 m/s, so the relative speed is 30 m/s and 300 m clears in 10 s.
Common trap. Subtracting the speeds as if the trains ran the same way — 60 − 48 = 12 km/h, or 10/3 m/s — gives 300 ÷ 10/3 = 90 s; trains running towards each other close at the sum, 108 km/h = 30 m/s, over the 300 m of both trains.