Two trains, one pole — Aptitude Question
Two trains, one pole: A train passes a pole in 15 seconds and a platform 100 m long in 25 seconds. Answer and worked solution on CodeKairo.
- Topic: Time, Speed & Distance
- Section: Quantitative Aptitude
- Difficulty: Hard
- Time target: 105 seconds
Question
A train passes a pole in 15 seconds and a platform 100 m long in 25 seconds. What is the length of the train?
Options
- 120 m
- 150 m
- 180 m
- 200 m
Show the answer and the worked solution
Correct answer: B. 150 m
Worked solution
- L = 15v and L + 100 = 25v.
- Subtracting: 100 = 10v → v = 10 m/s.
- L = 15 × 10 = 150 m.
Answer: 150 m.
The idea
Two crossing times give two equations in length and speed. Subtracting eliminates the length and hands you the speed immediately.
Shortcut. The platform adds 10 s to the pole's 15 s, so train : platform = 15 : 10 and the train is 1.5 × 100 = 150 m.
Common trap. Stopping at 25v = 25 × 10 = 250 m gives the train plus the platform; the train alone is what passes the pole, so its length is the 15 seconds' run at 10 m/s.