Question

find the time taken by a train of length 200 m to pass a stationary train of length 500 m if it is running at a speed of 70 km per hour

Answer 1

train had to cross 500m and its own length while crossing 500m train so total distance to be covered is 700m
time taken is 0.01hr
in seconds 
0.01*3600=36s

Answer 2

The train has to travel the distance which is equal to the length of object. Train also need to travel the distance of its own length.

This can be expalined as,
Suppose, the train is travelling. Its engine reached the object. Now the train has to cover the distance which is equal to the length of object. But now, only trains engine has crossed the object. The remaining part of train must also cross the object. So, we need to take the sum of lwngth of object an the length of train.

So, total distance travveled is 200+500=700m = 0.7 km
Speed=70 kmper hour
Speed = distance÷ time
So, time = distance ÷ speed.
So, time = 0.7÷70
So, time = 0.01 hours=0.01*3600 seconds=36 seconds