Two trains are running in the same direction at 40 km/h and 20 km/h. The fast train completely passes a man sitting in the slow train in 5 seconds. What is the length of the fast train (in meters)?

Introduction / Context:
Here the man is effectively stationary relative to the slow train, so the fast train’s relative speed with respect to the man equals the speed difference of the trains. Distance to pass the man equals the fast train’s length.

Given Data / Assumptions:

• v_fast = 40 km/h, v_slow = 20 km/h
• Time to pass = 5 s
• Same direction ⇒ relative speed = v_fast − v_slow

Concept / Approach:
Length = relative speed * time; convert relative speed to m/s first.

Step-by-Step Solution:

Verification / Alternative check:
5 s * 5.555... m/s = 27.78 m; option rounding matches 27 m.

Why Other Options Are Wrong:

• 232/9 ≈ 25.78 m, 277/9 ≈ 30.78 m, 23 m do not match 27.78 m.

Common Pitfalls:

• Adding speeds instead of subtracting for same direction.
• Forgetting to convert km/h to m/s.

Final Answer:
27 m