A man in a train notices that he can count 21 telephone posts in one minute. If they are known to be 25 meters apart, then at what speed is the train travelling?

Question: A man in a train notices that he can count 21 telephone posts in one minute. If they are known to be 25 meters apart, then at what speed is the train travelling?

Solution:
Number of gaps between 21 telephone posts = 20

∴ Distance travelled in 1 minute or 60 sec = (25 × 20) meter
= 500 m

∴ Speed of the train = (500 ÷ 60) m/sec
= 25/3 m/sec
= {(25/3) × (18/5)} km/h
= 30 km/h