Two trains of lengths 200 m and 300 m run on parallel tracks in opposite directions at 36 km/h and 54 km/h, respectively. How many seconds will they take to completely cross each other?

Introduction / Context:
For opposite-direction motion, relative speed is the sum of speeds. The distance to be covered for a complete pass is the sum of train lengths. Converting km/h to m/s ensures compatible units with metres and seconds for time calculation.

Given Data / Assumptions:

• Lengths: 200 m and 300 m ⇒ total = 500 m.
• Speeds: 36 km/h and 54 km/h.

Concept / Approach:
Relative speed = 36 + 54 = 90 km/h = 90 * (1000/3600) = 25 m/s. Time = distance / relative speed in SI units.

Step-by-Step Solution:

Verification / Alternative check:
Unit check: metres divided by metres/second yields seconds—correct dimensionally.

Why Other Options Are Wrong:
25/30/35 s correspond to lower relative speeds than 25 m/s, inconsistent with 36+54 km/h.

Common Pitfalls:
Forgetting to add speeds for opposite directions; mixing km/h directly with metres without conversion.

Final Answer:
20 sec