Two trains of lengths 512 m and 528 m run towards each other on parallel lines at 84 km/h and 60 km/h, respectively. From the instant they meet, in how many seconds will they be completely clear of each other?

Introduction / Context:
When two trains approach each other, their relative speed is the sum of their speeds. From the moment their fronts meet, they must collectively cover the sum of their lengths to be completely clear of one another.

Given Data / Assumptions:

• L1 = 512 m, L2 = 528 m.
• v1 = 84 km/h, v2 = 60 km/h.
• Uniform motion on parallel tracks.

Concept / Approach:
Total length to clear = L1 + L2. Relative speed v_rel = (v1 + v2) converted to m/s. Then time = (L1 + L2) / v_rel.

Step-by-Step Solution:

Verification / Alternative check:
At 40 m/s, covering 1040 m needs 26 s—consistent.

Why Other Options Are Wrong:
25 s or 27 s would imply non-integer effective speeds; 15 s is far too short.

Common Pitfalls:
Using difference of speeds (only for same-direction) or forgetting to convert km/h to m/s.

Final Answer:
26 s