Correct Answer - Option 3 : 3 : 1
Given:
Speed of train = 72 km/h
Time taken by train to cross a platform = 20 seconds
Time taken by train to cross a pole = 5 seconds
Formula:
Speed = distance/time
Distance = speed × time
Speed in km/h to m/s = multiply by 5/18
Calculation:
Speed of train in m/s = 72 × 5/18 = 20 m/s
Time taken by train to cross a pole = 5 seconds
Distance covered by train to cross a pole = The length of the train
Distance = 20 × 5 = 100 m
Distance covered in crossing the platform = 20 × 20 = 400m
Length of platform = 400 – 100 = 300 m
Ratio of length of platform to length of train = 300 : 100
∴ The required ratio is 3 : 1
