A 400 m long train passes a railway platform in 20 seconds with speed 90 km/hr. What is the length of platform?

100 m

Let the length of platform = x m

Length of train = 400 m.

Speed of train = 90 kmph = 90 × \(\frac{5}{18}\) m/sec = 25 m/sec.

Time = 20 sec

Distance traveled by train to cross the platform = Length of train + length of platform = 400 + x

Hence, 

Distance = Speed × Time

400 + x = 25 × 20 = 500 

x = 100 m