Equal lengths, different speeds — Aptitude Question
Equal lengths, different speeds: Two trains of equal length 120 m run on parallel tracks at 42 km/h and 30 km/h in… Answer and worked solution on CodeKairo.
- Topic: Time, Speed & Distance
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 90 seconds
Question
Two trains of equal length 120 m run on parallel tracks at 42 km/h and 30 km/h in the same direction. How long does the faster take to pass the slower?
Options
- 60 s
- 72 s
- 80 s
- 90 s
Show the answer and the worked solution
Correct answer: B. 72 s
Worked solution
- Relative speed = 42 − 30 = 12 km/h = 12 × 5/18 = 10/3 m/s.
- Distance = 240 m.
- Time = 240 ÷ (10/3) = 240 × 3/10 = 72 s.
Answer: 72 s.
The idea
Same direction, so subtract speeds, and the distance is both lengths together. Keeping 10/3 as a fraction avoids rounding.
Shortcut. 12 km/h is 12,000 m in 60 minutes, i.e. 200 m a minute, so the 240 m take 1.2 min = 72 s.
Common trap. 80 s — rounding the relative speed 10/3 m/s down to 3 m/s before dividing (240 ÷ 3); keeping the fraction gives 72 s.