Single correct option
Two trains of lengths 150 m and 250 m cross each other completely in 10 seconds when running in opposite directions. When running in the same direction, the faster train passes the slower one completely in 50 seconds. What is the speed of the faster train?
- A. 72 km/h
- B. 79.2 km/h
- C. 86.4 km/h
- D. 90 km/h
Show answer and solution
Answer: (C) 86.4 km/h
In both cases the distance to be covered is the combined length 150 + 250 = 400 m.
Opposite directions: a + b = 400/10 = 40 m/s.
Same direction: a − b = 400/50 = 8 m/s.
Adding: 2a = 48 → a = 24 m/s = 24 × 18/5 = 86.4 km/h (the slower train goes 16 m/s = 57.6 km/h).
Tip: Relative speed is the sum of the speeds for opposite directions and the difference for the same direction. The distance to clear is always the sum of the two lengths.
Opposite directions: a + b = 400/10 = 40 m/s.
Same direction: a − b = 400/50 = 8 m/s.
Adding: 2a = 48 → a = 24 m/s = 24 × 18/5 = 86.4 km/h (the slower train goes 16 m/s = 57.6 km/h).
Tip: Relative speed is the sum of the speeds for opposite directions and the difference for the same direction. The distance to clear is always the sum of the two lengths.
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