With what speed should a sound source move towards an observer so that the observer hears half the original frequency $(v)$?

$A$ bus is moving with a velocity of $5 \ m/s$ towards a huge wall. The driver sounds a horn of frequency $165 \ Hz$. If the speed of sound in air is $355 \ m/s$,the number of beats heard per second by a passenger on the bus will be:

$A$ train is moving along a semicircular track. The engine is at one end of the semicircular part of the track,while the last carriage is at the other end. The driver blows a whistle of frequency $200 \ Hz$. The velocity of sound is $340 \ m/s$. If the speed of the train is $30 \ m/s$,what is the apparent frequency observed by a passenger in the middle of the train (in $Hz$)?

$A$ source of sound whose frequency is $1000 \text{ Hz}$ is moving with a speed of $33 \text{ m/s}$. The waves reflected by a fixed obstacle are registered by a receiver that moves together with the source. If the speed of the sound waves is $330 \text{ m/s}$, then the frequency registered by the receiver is: (in $\text{ kHz}$)

An observer moves towards a stationary source of sound with a speed $\frac{1}{5}$th that of sound. The frequency of the sound emitted by the source is $f$. The apparent frequency recorded by the observer is

$A$ source of sound $A$ emitting waves of frequency $1800\,Hz$ is falling towards the ground with a terminal speed $v.$ The observer $B$ on the ground directly beneath the source receives waves of frequency $2150\,Hz.$ The source $A$ receives waves,reflected from the ground,of frequency nearly ..... $Hz$ (Speed of sound $= 343\,m/s$)