Problems on Trains – Two trains on parallel tracks, same direction: Train-1 is 200 m long at 54 km/h; Train-2 is 300 m long at 72 km/h. On parallel tracks and same direction, how long will they take to completely cross each other?

Introduction / Context:
For trains in the same direction, the relative speed equals the difference of their speeds. The distance to be covered for complete crossing is the sum of their lengths.

Given Data / Assumptions:

• L1 = 200 m at 54 km/h.
• L2 = 300 m at 72 km/h.
• Same direction on parallel tracks.

Concept / Approach:
Relative speed (m/s) = (72 − 54) km/h converted to m/s. Crossing distance = L1 + L2. Then time = distance / relative speed.

Step-by-Step Solution:
Relative speed = 18 km/h = 18 * (1000/3600) = 5 m/s.Distance = 200 + 300 = 500 m.Time = 500 / 5 = 100 s.

Verification / Alternative check:
Opposite-direction check would be much faster (sum of speeds); since here it is much slower, a large time like 100 s is reasonable.

Why Other Options Are Wrong:
90, 120, 150, 80 s do not match the computed distance/relative-speed ratio.

Common Pitfalls:
Adding speeds instead of subtracting for same direction; failing to convert km/h to m/s.

Final Answer:
100 sec