With what velocity an observer should move relative to a stationary source so that a sound of double the frequency of the source is heard by the observer?

$A$ source and an observer are moving towards each other with a speed equal to $\frac{v}{2}$,where $v$ is the speed of sound. The source is emitting sound of frequency $n$. The frequency heard by the observer will be

Two stationary sources each emit waves of wavelength $\lambda$. An observer moves from one source to the other with velocity $u$. The number of beats heard by the observer is:

$A$ bat is flitting about in a cave,navigating via ultrasonic beeps. Assume that the sound emission frequency of the bat is $40\; kHz$. During one fast swoop directly toward a flat wall surface,the bat is moving at $0.03$ times the speed of sound in air. What frequency (in $kHz$) does the bat hear reflected off the wall?

Two men are walking along a horizontal straight line in the same direction. The man in front walks at a speed of $1.0 \,m \,s^{-1}$ and the man behind walks at a speed of $2.0 \,m \,s^{-1}$. $A$ third man is standing at a height of $12 \,m$ above the same horizontal line such that all three men are in a vertical plane. The two walking men are blowing identical whistles which emit a sound of frequency $1430 \,Hz$. The speed of sound in air is $330 \,m \,s^{-1}$. At the instant when the moving men are $10 \,m$ apart,the stationary man is equidistant from them. The frequency of beats in $Hz$ heard by the stationary man at this instant is:

$A$ driver in a car,approaching a vertical wall,notices that the frequency of his car horn has changed from $440 \, Hz$ to $480 \, Hz$ when it gets reflected from the wall. If the speed of sound in air is $345 \, m/s,$ then the speed of the car is $....... \, km/hr$.