Find ambulance speed from Doppler shift at 480 Hz and 420 Hz

[asdf12321asdf]

Homework Statement

A stationary observer at a crosswalk hears an ambulance siren with an apparent frequency of
480 Hz when the ambulance is approaching. After the ambulance passes the apparent frequency is only 420 Hz. Find the speed of the ambulance. Assume v = 343 m/s for the speed of sound in air.

Homework Equations

source moving towards observer: f' = (vf)/(v - v_s)
source moving away from observer: f' = (vf)/(v + v_s)

The Attempt at a Solution

I am having trouble because I am not sure how to figure out what the frequency of the sound given off by the ambulance is. I tried using 450 since it is halfway between, and it gave me a close answer, but not the correct one. Any help would be much appreciated.

Thanks a lot

 

[tiny-tim]

hi asdf12321asdf!

source moving towards observer: f' = (vf)/(v - v_s)
source moving away from observer: f' = (vf)/(v + v_s)

The Attempt at a Solution

I am having trouble because I am not sure how to figure out what the frequency of the sound given off by the ambulance is. I tried using 450 since it is halfway between, and it gave me a close answer, but not the correct one.

Quelle surprise!

c'mon … use some algebra! ​

 

[asdf12321asdf]

oh ok I figured it out thanks a lot