When a car slams on its brakes and starts to skid, it leaves skid marks on the road. Investigators use these marks to determine how fast a car was going when it started to brake. The car's speed,
v
v
, (in feet per second) is related to the skid mark length,
d
d
, (in feet) by the equation
v
=
√
32
f
d
v
=
32
f
d
, where
f
f
is the coefficient of friction determined by the material the car was skidding on.

Wet asphalt has a coefficient of friction of 0.5. If a car was driving 40 miles per hour and slams on its brakes, how long of a skid mark will it leave? 1 mile = 5280 feet.