Frequency involving moving observer and source

[n77ler]

Homework Statement

You are driving along a highway at 27.5 m/s when you hear the siren of a police car approaching you from behind and you perceive the frequency as 1250 Hz. You are relieved that he is in pursuit of a different speeder when he continues past you, but now you perceive the frequency as 1202 Hz. What is the speed of the police car? The speed of sound in air is 343 m/s.

So, U_o= 27.5 m/s
U_s = ??
v= 343 m/s

Homework Equations

It states that the perceived frequency from behind is 1250 ad the 1202 after it is gone by

The Attempt at a Solution

I used the equation f ' =(1+- u_o/v)/(1+-u_s/v)f
Because it gives you information when the car is behind you and then after it has passed you I used two equations and let them equal each other but I am unsure if the perceived frequencies are f or f ' ??

 

[n77ler]

Ok so I've wrote out my equation but I'm finding it really hard to get through because I don't know which givens go with the variables in my equations... In my equation the left side is the first part of questions and right side is the second side

((1 - u_o/v)/(1 - u_s/v))f - f ' = ((1 + u_o/v)/(1+ u_s))f - f '

 

[n77ler]

anybodyy ? :(