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?
Distance between 1st and 21st post = $$(21-1)\times50=1000$$ m = $$1$$ km
Thus, the train travels 1 km in 1 minute.
=> Speed of train, i.e. distance travelled in 60 minutes = $$1\times60=60$$ km/hr
=> Ans - (B)
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