Two trains of equal length run on parallel lines in the same direction at 46 km/h and 36 km/h. The faster completely passes the slower in 36 seconds. What is the length of each train?

Introduction / Context:
For same-direction overtaking, the relative speed is the difference of speeds. The distance that must be covered to “completely pass” equals the sum of both train lengths. With equal lengths, that distance is 2L.

Given Data / Assumptions:

• Speeds: 46 km/h and 36 km/h (same direction).
• Pass time = 36 s.
• Lengths are equal = L each.

Concept / Approach:
Relative speed v_rel = (46 − 36) km/h = 10 km/h = 2.777... m/s. Distance to pass = 2L = v_rel * t.

Step-by-Step Solution:
2L = 2.777... * 36 = 100 m → L = 50 m.

Verification / Alternative check:
At 2.777... m/s, covering 100 m requires 36 s, as given.

Why Other Options Are Wrong:
72, 80, 82 m contradict the product of relative speed and time with equal-length assumption.

Common Pitfalls:
Using only one length instead of both in same-direction overtaking, or adding speeds (used for opposite direction).

Final Answer:
50 metres