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

**∴** **T****he required ratio is 3 : 1**