The cost of gasoline per hour for running a certain car is directly proportional to the square of the speed. I f the cost is $2.40 per hour for a speed of 40 km/h, what is the cost per hour for a speed of 60 km/h?
Multiply the 40 mph cost (2.40/hr) by (60/40)^2.
The cost per mile will decrease by a smaller factor (3/2), since the car goes farther per mile.