The formula v = sqrt 20L can be used to estimate the speed of a car...?

V in MPH, based on the length, L, in feet, of it's skid marks upon sudden braking on a dry asphalt road. If a car involved in an accident and its skid mark measures 405 feet, at what estimated speed was the car traveling when it applied its brakes just prior to the accident.

When I started to solve this problem I started as sqrt of 20(405) which is the sqrt of 8100 which = 90. So I'm thinking 90 MPH is the answer to this question can you please let me know if I did this right and if I didn't explain to me how to get the right answer.