Question

A stationary observer heard an ambulance’s frequency 25 Hz lower than the 750 Hz at which it is emitted. If the air temperature is 20.0°c, how fast is the ambulance going?

Answer 1

The Doppler's effect equation gives

$$\Delta f=f_0\frac{v}{c}$$

Hence,

$$v=c\frac{\Delta f}{f_0}=\rm 340\: m/s\times\frac{25}{750}=11.3\: m/s$$