To calculate the speed of the train, we can use the information that the man can count 21 telephone posts in one minute and that the posts are known to be 50 meters apart. This means that in one minute, the train travels a distance equal to the spacing between 21 telephone posts.

Distance traveled in one minute = 21 posts * 50 meters/post = 1050 meters

Since there are 60 seconds in a minute, we need to convert the distance to meters per second:

Speed (in meters per second) = Distance (in meters) / Time (in seconds) Speed = 1050 meters / 60 seconds = 17.5 meters/second

To express the speed in kilometers per hour (km/h), we can multiply by 3.6 (since 1 meter per second is equal to 3.6 kilometers per hour):

Speed = 17.5 meters/second * 3.6 km/h = 63 km/h

So, the speed of the train is 63 kilometers per hour.