The respective ratio between the speed of a car , a train , and a bus is 5 : 9 : 4 . The average speed of the car, bus and train is 72 km / hr together . what is the average speed of the car and the train together?

Correct Answer: 84 km / hr
Let the speeds of the car, train and bus be 5x, 9x and 4x km/hr respectively. Average speed = (5x + 9x + 4x)/3 = 18x /3 = 6x km/hr. Also, 6x = 72 = > x = 12 km/hr Therefore, the average speed of the car and train together is = ( 5x + 9x)/2 = 7x = 7 x 12 = 84 km/hr.