The length of the bridge, which a train 400 metres long and travelling at 60 km/hr can cross in 36 seconds, is:

First, convert the speed of the train from km/hr to m/s:
Speed=60×
18
5
​
=
3
50
​
 m/s
Let the length of the bridge be B metres.
When crossing a bridge, the total distance covered by the train is equal to the length of the train plus the length of the bridge:

Distance=Length of train+Length of bridge=400+B
Using the formula Distance=Speed×Time:

400+B=
3
50
​
×36
400+B=50×12
400+B=600
B=600−400=200 m
Thus, the length of the bridge is 200 m.