Problems on Trains – Time to pass a telegraph post: A train 135 m long runs at 54 km/h. In what time will it pass a telegraph post completely?

Introduction / Context:
Passing a single post requires the train to traverse its own length at its steady speed. Use consistent units to avoid calculation errors.

Given Data / Assumptions:

• L_train = 135 m.
• Speed = 54 km/h = 15 m/s.
• Motion is uniform.

Concept / Approach:
Time t = distance / speed = L_train / v, with v in m/s.

Step-by-Step Solution:
v = 54 * (1000/3600) = 15 m/s.t = 135 / 15 = 9 s.

Verification / Alternative check:
Reverse: 9 s * 15 m/s = 135 m, which equals the train length.

Why Other Options Are Wrong:
12, 8, 6, 10 s do not equal 135/15.

Common Pitfalls:
Leaving speed in km/h or dividing by 54 directly without conversion.

Final Answer:
9 sec