Speed of car with sound I've got this task:

A traffic policeman standing on the highway notices a change in tone of exactly one major third (f2/f1 = 4/5).
What speed can he infer? Speed of sound c = 340 m/s

I've got this attempt: 340 m/s * (1+√5/4) = 765m/S = 2754 km/h
Can someone tell me if this is correct or if I'm completely wrong?
Hints

Define the frequency of the car as $f_0$, the heard frequency when it's approaching as $f1$ and the heard frequency when it's departing as $f_2$
Write the Doppler formula for both $f_1$ and $f_2$ using a receiver velocity of 0 (since the policeman is stationary with respect to the air) and a car velocity of $v_s$. Pay attention to the sign of the source velocity.
Write the ratio of both formulas and equal it to a major third. $f_0$ should drop out and you are left with a single equation for $v_s$
Solve for $v_s$