Answer:
f (frequency) = V / y where V is the speed of sound and y the wavelength
f = 1500 m/s / 1.5 m = 1000 / sec
T (period) = 1 / f = .001 sec
Suppose you replace the horn by a drum then the period would be the time between the beats of the drum - now if the source is moving towards the observer then the distance between crests of the wave produced by the drum will be shortened by V * T because of the motion of the drum "towards" the observer, and since the wavelength is shorter the frequency heard by the observer will be higher, and the higher the speed of of the car the shorter the wavelength as seen by the observer and the higher the frequency.
Also, if the car is moving away from the observer then the distance between the crests of the wave emitted will be further apart, and the observer will hear a lower frequency.
