A highway engineer wants to estimate the maximum number of cars that can safely travel on a particular road at a given speed. She assumes that each car is 15 feet long, travels at speed s, and follows the car in front of it at a safe distance for that speed. She finds that the number N of cars that can pass a given spot per minute is modeled by the function N(s)
N (s) = (81s) /(17+17(s/20)^2)
At what speed can the greatest number of cars travel safely on thatroad?