The distance it takes an automobile to stop varies directly as the square of its speed. Thus, by the definition of direct proportionality, d=kx2, in which d is the stopping distance in feet, x is the speed of the car in miles per hour, and k is a constant.d=kx2320=k(40)2320=1600k1600320​=kd=1600320​x2d=1600320​(50)2d=500