A person travelling in a car moving at 72 km/h blows a whistle that...

Speed of the car, $v_s = 72 \text{ km/h} = 72 \times \dfrac{5}{18} \text{ m/s} = 20 \text{ m/s}$

Speed of sound, $v = 340 \text{ m/s}$

Actual frequency, $f = 1250 \text{ Hz}$

Since the car is moving away from the stationary observer, the apparent frequency $f'$ is given by the Doppler effect formula:

$f' = f \left( \dfrac{v}{v + v_s} \right)<br><br>$f' = 1250 \left( \dfrac{340}{340 + 20} \right)

$f' = 1250 \left( \dfrac{340}{360} \right)$

$f' = 1250 \times \dfrac{17}{18}$

$f' \approx1180.55 \text{ Hz} \approx1181 \text{ Hz}$