The speed a vehicle was traveling, s, in miles per hour, when the brakes were first applied, can be estimated using the formula:

where d is the length of the vehicle's skid marks, in feet.

a) Determine the speed of a car that made skid marks 4ft long.

b) Determine the speed of a car that made skid marks 16ft long.

c) Determine the speed of a car that made skid marks 256ft. long.