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, at what speed is the train travelling?  Select the correct option
A) 61 km/hr  B) 56 km/hr  C) 63 km/hr  D) 60 km/hr  E) 70 km/hr
21 telephone posts in 1 min   No of gaps b/w 21 posts = 20   adjacent post = 50 m     20*50= 
<gadget id="calculator">  20*50</gadget>
<output>1_000</output>
1000 m in 1 min   dist= 1000 m = 1 km   time = 1 min = 1/60 hr   speed = 1/(1/60)= 
<gadget id="calculator">1/(1/60)</gadget>
<output>60</output>
60 km/hr   ANSWER 60 km/hr
<result>D</result>
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, at what speed is the train travelling?