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 for same direction
v_rel = 40 − 20 = 20 km/hrConvert to m/s: 20 × (5/18) = 5.5556 m/s

Step 2: Distance while passing the man
Passing a man ⇒ distance = length of the train (L)L = v_rel × time = 5.5556 × 5 = 27.7778 m ≈ 27.78 m

Checks & Common Pitfalls

• Same direction ⇒ subtract speeds, not add.
• When passing a man, use only the moving train's length as distance.

Final Answer
The fast train's length is 27.78 m.