Discussion
A 240-m-long train passes a pole in 18 seconds and completely passes another train running in the opposite direction at a speed of 42 km/h in 17 seconds. What is the length of the other train?
Answer: B
The other train is 185 m long. The first train covers its own length of 240 m past the pole in 18 s, so its speed is \(\frac{240}{18} = \frac{40}{3}\) m/s, which is 48 km/h. The trains move in opposite directions, so the relative speed is \(48 + 42 = 90\) km/h, which is 25 m/s. In 17 s they cover \[ 25\times 17 = 425\text{ m} \] which is the sum of the two lengths. The other train is \(425 - 240 = 185\) m. Using the difference of speeds (as for the same direction) would give a different length, so 170, 195 and 200 do not arise.
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