Discussion
Two trains 140 and 100 metres long, are going in the same direction. The faster train takes one minute to pass the other completely. If they are moving in the opposite directions, they pass each other completely in 6 seconds. The speed of the faster train is :
Answer: B
The faster train's speed is 22 m/sec.
To pass each other completely, the trains must cover their combined length, \(140 + 100 = 240\) m. In the same direction they close at the difference of their speeds, and in opposite directions at the sum:
\[ \begin{aligned} u - v &= \frac{240}{60} = 4\ \text{m/s} \\ u + v &= \frac{240}{6} = 40\ \text{m/s} \end{aligned} \]
Adding, \(2u = 44\), so \(u = 22\) m/s, and the slower train runs at 18 m/s.
20 m/s is the average of the two speeds, and 42 and 28 m/s do not satisfy both equations.
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