the cost of fuel for running a train is propotional to the square of the speed it generates in km per hour. It costs Rs. 750 per hour at a speed of...

the cost of fuel for running a train is propotional to the square of the speed it generates in km per hour. It costs Rs. 750 per hour at a speed of 60 km per hour. Find the most economical speed if the fixed caharges are Rs. 1000 per hour