A train 550 metres long is running at a speed of 55 km/h. In how many seconds will it completely pass a signal post placed at the track-side (i.e., clear the post with its entire length)?

Introduction / Context:
To pass a signal post, a train must cover a distance equal to its own length. Hence the required time depends only on the train’s length and its speed in m/s.

Given Data / Assumptions:

• Train length L = 550 m.
• Speed v = 55 km/h.
• Signal post has no length; distance to cover = L.

Concept / Approach:
Convert km/h to m/s using 5/18: v(m/s) = v(km/h) * 5/18. Then time t = distance / speed = L / v.

Step-by-Step Solution:

Verification / Alternative check:
Proportionally, doubling length or halving speed would double time. Units are consistent (metres over metres/second gives seconds).

Why Other Options Are Wrong:
24, 30, 42, 48 seconds do not match L/v after the correct unit conversion.

Common Pitfalls:
Forgetting to convert to m/s; using the bridge/platform formula instead of post (only train length counts here).

Final Answer:
36 sec