Answer:985.71 Hz
Explanation:
Given
Speed of ambulance (source) [tex]v_s=108\ kmph\approx 30\ m/s[/tex]
Original Frequency of sound wave [tex]f=900\ Hz[/tex]
speed of sound waves [tex]v=345\ m/s[/tex]
According to Doppler apparent frequency will be different to original frequency when speed of source is moving w.r.t to observer.
[tex]f'=f(\frac{v+v_o}{v-v_s})[/tex]
where f'=apparent frequency
f=original frequency
[tex]v_o[/tex]=observer speed
[tex]f'=900\times (\frac{345+0}{345-30})[/tex]
[tex]f'=900\times (\frac{345}{315})[/tex]
[tex]f'=985.71\ Hz[/tex]
