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

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

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

Actual frequency of the whistle, $f_0 = 1250 \text{ Hz}$

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

f = f_0 \left(\dfrac{v}{v - v_s} \right)

Substituting the given values:

$f = 1250 \left( \dfrac{340}{340 - 20} \right)$

f = 1250 \left(\dfrac{340}{320} \right)

$f = 1250 \times$\dfrac{17}{16}

$f = 1328.125 \text{ Hz}$

The frequency heard by the person is approximately $1328 \text{ Hz}$.