Here the trick is :
Let us consider a distance frame in which we have a bus followed by a gap after bus followed by car followed by a gap after car..
So the distance in this frame = 20 + 10 + 15 + 5
= 50 m
Now given speed = 36 km / hr
= 36 * 5 / 18 m/s
= 10 m/s
Hence time taken to cover 50 m = 50 / 10
= 5 s
In other words , one bus and one car is involved in this time frame
Hence in 5s , no of vehicles passed = 2
in 1s , no of vehicles passed = 2/5
==> in 1 hr i.e. in 3600 s , no of vehicles passed = 2/5 * 3600
= 1440
Hence the correct answer should be 1440..Hence option A)