Two trains 70 m and 80 m long run on parallel tracks in opposite directions at 68 km/h and 40 km/h, respectively. How many seconds will they take to completely pass each other?

Introduction / Context:
For two trains moving in opposite directions, relative speed is the sum of speeds. Distance to be covered to cross fully equals the sum of their lengths.

Given Data / Assumptions:

• L1 = 70 m; L2 = 80 m → total S = 150 m.
• v1 = 68 km/h; v2 = 40 km/h; opposite directions.

Concept / Approach:
v_rel = (68 + 40) km/h = 108 km/h. Convert v_rel to m/s; t = S / v_rel_mps.

Step-by-Step Solution:

Verification / Alternative check:
Had they moved in the same direction, relative speed would be 28 km/h and the time would be much larger—this quick 5 s is consistent with opposing motion.

Why Other Options Are Wrong:
6, 8, 10, 12 seconds arise from wrong relative speed or length sums.

Common Pitfalls:
Forgetting to add lengths; using difference of speeds in opposite direction cases.

Final Answer:
5 seconds