Two trains are running at 40 km/hr and 20 km/hr respectively in the same direction. Fast train completely passes a man sitting in the slower train in 5 seconds. What is the length of the fast train?

Given Data

• Speeds of trains (same direction) = 40 km/hr and 20 km/hr
• Fast train completely passes a man sitting in the slower train in 5 s
• Required: length of the fast train

Step 1: Relative speed (same direction ⇒ difference)v_rel = 40 − 20 = 20 km/hrConvert to m/s: 20 × (5/18) = 5.5556 m/s

Step 2: Distance covered while passing the manWhen passing a man, distance = length of the passing train (L).L = v_rel × time = 5.5556 × 5 = 27.7778 m ⇒ 27.78 m (rounded to 2 decimals)

Checks & Common Pitfalls

• Same direction uses the difference of speeds, not the sum.
• For a man/passenger, distance equals only the train length.
• Keep units consistent and round to two decimals if needed.

Final AnswerThe length of the fast train is 27.78 m.