A car moves towards a wall at a speed of 5 ms^-1 with its horn blowing. The driver inside the car hears 5 beats per second. If the speed of sound in air is 350 ms^-1, the actual frequency of the horn is

The driver will listen two sounds. Direct sound of the horn and sound reflected from the wall. For the reflected sound, the wall acts assource (virtual source). When the car moves towards the wall the virtual source also appears to move towards the car with the same speed. Relative velocity of the car with respect to the wall (virtual source) is ${\upsilon }_R=2{\upsilon }_s=2\times 5=10m{s}^{-1}$
$V^{\text{'}}=V\left[\frac{\upsilon }{\upsilon -{\upsilon }_R}\right]v=v\left[\frac{350}{350-10}\right]$  but 

$v-v=5\therefore \frac{350}{340}v-v=5\Rightarrow v=170 Hz$