Train–tunnel clearance time A train 540 m long runs at 72 km/h. In what time (in seconds) will it pass completely through a tunnel 160 m long?

Introduction / Context:
When a train passes a tunnel, the tail must clear the far end. Thus, the train must cover its own length plus the tunnel length. Consistent unit conversion is key to accuracy.

Given Data / Assumptions:

• Train length = 540 m.
• Tunnel length = 160 m.
• Speed = 72 km/h (uniform).

Concept / Approach:
Use distance = speed * time. Convert 72 km/h to m/s to align with meters and seconds.

Step-by-Step Solution:

Verification / Alternative check:
Multiply back: 35 s * 20 m/s = 700 m which equals 540 + 160, confirming correctness.

Why Other Options Are Wrong:
30 s, 40 s, 42 s: Do not match the exact D/v. Only 35 s equals the precise computation.

Common Pitfalls:
Forgetting to add tunnel length or using km/h directly with meters/seconds. Always convert 72 km/h to 20 m/s.

Final Answer:
35 sec