A train 240 m long passes a bridge 120 m long in 24 seconds. Find the train’s speed in km/h.

Introduction / Context:
When crossing a bridge/platform, the train must cover its own length plus the structure’s length. Divide total distance by time to get speed, then convert to km/h if needed.

Given Data / Assumptions:

• L_train = 240 m.
• L_bridge = 120 m.
• t = 24 s.

Concept / Approach:
Total distance S = 240 + 120 = 360 m. Speed v(m/s) = S / t; then convert to km/h by multiplying by 3.6.

Step-by-Step Solution:

Verification / Alternative check:
If time were halved to 12 s for same distances, speed would double to 108 km/h—consistent with proportionality.

Why Other Options Are Wrong:
36, 42, 45, 48 km/h arise from mis-adding lengths or conversion errors.

Common Pitfalls:
Forgetting to add both lengths; mixing up 3.6 vs 5/18 conversion factors.

Final Answer:
54 km/h