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A man in a train notices that he can count 21 telephone posts in one minute. If they are known to be 50 metres apart, then at what speed is the train travelling?

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57 km/hr.

60 km/hr.

63 km/hr.

55 km/hr.

answer is B.

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Detailed Solution

Given,
Distance between telephone posts = 50 meters.
Total telephone posts = 21
Now, in between the 21 telephone posts the gap is,
Distance

= 50 \times 20

Distance = 1000 m

Distance

= 1 km

(since, 1 km = 1000 m)
Now time taken to cover this distance,

Time = 1 \min = \frac{1}{60} hr

(since, 1 hr = 60 min)
Suppose speed of train is x.
Now we know that,

x = \frac{Distance}{time}

= \frac{1 km}{\frac{1}{60} hr}

x = 60 \frac{km}{hr}

Hence, option 2 is correct.

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