A train moving with a speed 54 km/h crosses a platform and a man on the platform in 36 seconds and 20 seconds respectively. How long is the platform?

Correct option is B

Given:​
Speed of the train = 54 km/h
Time to cross the man = 20 seconds
Time to cross the platform = 36 seconds
Formula Used:
Distance = Speed × Time
Speed in m/s = Speed in km/h × 5/18
Solution:

Speed of train is 54 km/hr and let length of the train and platform be x m and y m respectively.

As we know,
Speed = distance/time
=> 54 × 5/18 = x/20
=> x = 15 × 20
=> x = 300
Again,
=> 54 × 5/18 = (x + y)/36
=> 15 × 36 = 300 + y
=> y = 540 – 300
=> y = 240 m
Alternate Method:

Length of the platform = 54 × 5/18 × (36 – 20) = 15 × 16 = 240 m