What Speed is the Racecar Moving?

[dmolson]

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?

a. 68 m/s
b. 49 m/s
c. 61 m/s
d. 72 m/s

 

[dmolson]

I cannot figure out how to work this problem out, any help would be great. Thanks in advance.

 

[QuantumCrash]

Think of what formulae that comes to your mind when you do this question.