A person travels from Jaipur to Nagpur which is 450 km by three different vehicles. He travels 150 km by bus with a speed of 30 km / h and then waited for 20 minutes for the train and then travels another 150 km by train at a speed of 75 km / h and then waited for 40 minutes to travel further journey by car at a speed of 40 km / h to the final destination, find the average speed of the journey?