The cost of running a bus from A to B, is Rs. (av+vb) , where v km/h is the average speed of the bus. When the bus travels at 30 km/h, the cost comes out to be Rs. 75 while at 40 km/h, it is Rs. 65. Then the most economical speed (in km/h) of the bus is : [JEE-MAIN ONLINE-2013]
(1) 40
(2) 60
(3) 45
(4) 50