The speed s in miles per hour that a car is traveling when it goes into a skid can be estimated by the formula s = √ 30fd, where f is the coefficient of friction and d is the length of the skid marks in feet. On the highway near Lake Tahoe, a police officer finds a car on the shoulder, abandoned by a driver after a skid and crash. He is sure that the driver was driving faster than the speed limit of 20 mi/h because the skid marks
measure 9 feet and the coefficient of friction under those conditions would be 0.7. At about what speed was the driver driving at the time of the skid? Round your answer
to the nearest mi/h.
A. 23 mi/h
B. 189 mi/h
C. 14 mi/h
D. 19 mi/h