A toy-car, blowing its horn, is moving with a steady speed of $5 m/s$, away from a wall. An observer, towards whom the toy car is moving, is able to hear $5$ beats per second. If the velocity of sound in air is $340 m/s$, the frequency of the horn of the toy car is close to :

Question

$\begin{array}{1 1}(1) 680Hz \\ (2) 510Hz \\ (3) 340 Hz \\ (4)170Hz \end{array}$