A train 75 m long runs at 20 km/h. In how many seconds will it pass a stationary man standing beside the track?

Introduction / Context:
Passing a stationary observer requires the train to cover its own length relative to the observer. Use speed converted to m/s for the calculation with seconds.

Given Data / Assumptions:

• Length L = 75 m.
• Speed v = 20 km/h.
• Observer is stationary (speed = 0).

Concept / Approach:
Time = distance / speed, in consistent units (m and s). Convert 20 km/h to m/s.

Step-by-Step Solution:
20 km/h = 20 * 1000/3600 = 5.555... m/s.t = 75 / 5.555... ≈ 13.5 s.

Verification / Alternative check:
5.555... m/s for 13.5 s covers 75 m exactly.

Why Other Options Are Wrong:
12, 14, or 15 s correspond to speeds significantly different from 20 km/h for a 75 m length.

Common Pitfalls:
Forgetting to convert km/h to m/s before dividing by seconds.

Final Answer:
13.5 seconds