Please help!!! Math!!!

Question

The stopping distance d of a car after the brakes have been applied varies directly as the square of the speed r. If a car traveling 40 mph can stop in 80 ​ft, how fast can a car travel and still stop in 320 ​ft?

Answer 1

d = kr^2

if you stop in 80 feet going 40mph then

80 =k(40)^2

k=80/1600 = 1/20

d = (1/20)r^2

r^2 = 20d

r^2 =20(320) = 6400

r = sqr6400

r = 80 mph is the speed where you can stop in 320 feet