Correct Answer - Option 3 : 60 km/hr
Given:
A man in a train notices that he can count 21 telephone posts in one minute
The telephone post is 50 m apart
Formula used:
Speed = Distance/Time
The total distance between n objects = (n – 1) × d
where d is the distance between the two objects
Calculation:
The total distance between 21 telephone posts
⇒ (21 – 1) × 50
⇒ 1 km
Time into hours
⇒ 1 min = 1/60 hour
Speed of the train = [1/(1/60)] km/hr
⇒ 60 km/hr
∴ The speed of the train is 60 km/hr
