A train of length 150 m passes another train of length 100 m coming from the opposite direction in 10 seconds. If the first train's speed is 30 km/h, what is the speed of the second train?

Introduction / Context:
Opposite-direction crossing uses sum of speeds as relative speed. The combined length must be covered in the given time. Knowing one speed yields the other.

Given Data / Assumptions:

• L1 = 150 m at 30 km/h; L2 = 100 m at unknown speed.
• Opposite directions; crossing time = 10 s.

Concept / Approach:
Relative speed v_rel (m/s) = (L1 + L2)/t = 250/10 = 25 m/s. Convert 30 km/h to m/s and subtract from v_rel to get the second train's speed in m/s, then km/h.

Step-by-Step Solution:
30 km/h = 8.333... m/s.Second speed = 25 − 8.333... = 16.666... m/s = 60 km/h.

Verification / Alternative check:
Total km/h = 30 + 60 = 90 km/h = 25 m/s; times 10 s gives 250 m, the combined length.

Why Other Options Are Wrong:
54 or 72 km/h would not give 25 m/s combined; 36 km/h is too slow.

Common Pitfalls:
Using difference of speeds instead of sum for opposite directions.

Final Answer:
60 km/h