Coefficient of kinetic friction, Physics problem?

A rough rule of thumb used by the police states that the frictional force between dry concrete and a skidding car's tire is about nine-tenths of the car's weight. If the skid marks left by a car in coming to rest are 20m long, about how fast was the car going just before the brakes were applied? In this rule, what is the umed coefficient of kinetic friction?