Discussion
A train can cross a pole in 15 seconds and travelling at the same speed it can cross a bridge of 500 m in 45 seconds, then find the length of the train (in metre).
Answer: A
The train is 250 m long.
Crossing a pole, the train covers its own length \(L\) in 15 s. Crossing the bridge, it covers \(L + 500\) in 45 s at the same speed.
\[ \begin{aligned} \frac{L}{15} &= \frac{L + 500}{45} \\ 3L &= L + 500 \\ L &= 250\ \text{m} \end{aligned} \]
Check: speed is \(250 / 15 = 16.67\) m/s, and \(750 / 16.67 = 45\) s for the bridge. The bridge takes three times as long, so it adds twice the train's length.
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