Sound of the motor of a racecar?

Question

The pitch of the sound from a racecar engine drops the musical interval of a fourth when it passes the spectators. This means the frequency of the sound after passing is 0.75 times what it was before. How fast is the racecar moving?

Answer 1

The standard speed of sound thru air is 770 mph.
The speed of the car is x.

If the pitch drops by 25% then (770 - x)/(770 + x) = .75

770 - x = 577.5 + .75x

192.5 =1.75 x

110 mph = x