Slippery Roads The coefficient of friction x measures the friction between the tires of a car and the road, where 0 < x ≤ 1. A smaller value of x indicates that the road is more slippery. If a car is traveling at 60 miles per hour, then the braking dis- tance D in feet is given by the formula D(x) = 120/x. 

(a) What happens to the braking distance as the coefficient of friction becomes smaller? 

(b) Find values for the coefficient of friction x that correspond to a braking distance of 400 feet or more.