Two trains run in the same direction on parallel tracks. Lucknow Mail is 120 m long at 40 km/h; Punjab Mail is 80 m long at 50 km/h. How many seconds will they take to completely cross each other (overtaking, same direction)?

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

Given Data / Assumptions:

• Lengths: 120 m and 80 m ⇒ total = 200 m.
• Speeds: 40 km/h and 50 km/h (same direction).

Concept / Approach:
Relative speed = 50 − 40 = 10 km/h. Convert to m/s, then time = distance / speed.

Step-by-Step Solution:

Verification / Alternative check:
2.777... m/s * 72 s = 200 m, exactly the combined length.

Why Other Options Are Wrong:
8/11.5/12.5 s require much larger relative speeds than 10 km/h.

Common Pitfalls:
Using sum of speeds instead of difference for same-direction motion.

Final Answer:
72 sec.