Una ambulancia se desplaza hacia el norte a 23,5 m/s. su sirena emite un sonido de 500hz de frecuencia. calcular la frecuencia aparente que percibe un observador que viaja en su auto, también hacia el norte , a 28,6, cuando:

(I attach your complete question in the picture below)

The apparent frequency would be  

f' = 493.26 Hz

Step-by-step explanation:

This a Doppler's effect problem. This effects produces a slight change in the frequency of a traveling wave, due to the effect of the velocity of the receiver (observer).

The formula used to describe the change, as both vehicles are moving away of each other:

f' = [ (Vwave + Vobserver) / (Vwave + Vsource ) ]*f

Where  

Vwave = 340 m/s ( speed of sound)

Vobserver = 18.6 m/s (observer = car)

Vsource = 23.5 m/s (speed of the ambulance = source)

f = 500 Hz (frequency of the sound)

We just need to substitute in the equation

f' = [(340+ 18.6) / (340+ 23.5)] * 500 Hz

f' = [0.9865] * 500 Hz = 493.26 Hz

f' = 493.26 Hz

i believe the correct answer is b.