Discussion
If a train passes an electrical pole in 30 seconds and passes a platform of length 200m in 50 seconds, the length of the train is
Answer: B
The train is 300 m long.
Passing a pole, the train covers its own length \(L\) in 30 s. Passing the platform, it covers \(L + 200\) in 50 s, at the same speed.
\[ \begin{aligned} \frac{L}{30} &= \frac{L + 200}{50} \\ 50L &= 30L + 6000 \\ L &= 300\ \text{m} \end{aligned} \]
Shortcut: the extra 20 seconds are spent covering the 200 m platform, so the speed is 10 m/s, and in 30 s the train covers 300 m. 320, 280 and 250 m do not give the same speed in both cases.
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