Question : A train crosses a stationary pole in 20 seconds and a bridge in 32 seconds. If the length of the bridge is 1200 metres, then what is the speed of the train?
Option 1: 360 km/hr
Option 2: 180 km/hr
Option 3: 270 km/hr
Option 4: 450 km/hr
Correct Answer: 360 km/hr
Solution :
A train crosses a stationary pole completely in = 20 seconds and
The train crosses a bridge completely in = 32 seconds.
The length of the bridge is = 1200 metres
Let the length and speed of the train be L meters and x m/s respectively.
According to the concept,
While a train crosses a pole, it crosses its own length.
20 x = L....(1)
While a train crosses a bridge, it crosses the sum of its length and the length of the bridge.
32x = (L + 1200)
⇒ 32x = 20x + 1200 (From 1)
⇒ 12x = 1200
⇒ x = 100 metres = 100 × $\frac{18}{5}$ = 360 km/hr
Hence, the correct answer is 360 km/hr.
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