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

An observer is standing $500 \,m$ away from a vertical hill. Starting between the observer and the hill, a police van sounding a siren of frequency $1000 \,Hz$ moves towards the hill with a uniform speed. If the frequency of the sound heard directly from the siren is $970 \,Hz$, the frequency of the sound heard after reflection from the hill (in $Hz$) is about, (velocity of sound $= 330 \,m/s$):

$A$ whistle of frequency $540 Hz$ rotates in a horizontal circle of radius $2 m$ at an angular speed of $15 rad/s$. What is the highest frequency heard by a listener at rest with respect to the center of the circle (in $Hz$)? (Velocity of sound in air $= 330 m/s$)

$A$ whistle giving out $450 Hz$ approaches a stationary observer at a speed of $33 m/s$. The frequency heard by the observer in $Hz$ is (Take speed of sound $v = 330 m/s$)

$A$ and $B$ are two sources generating sound waves. $A$ listener is situated at $C$. The frequency of the source at $A$ is $500 \, Hz$. $A$ now moves towards $C$ with a speed of $4 \, m/s$. The number of beats heard at $C$ is $6$. When $A$ moves away from $C$ with a speed of $4 \, m/s$,the number of beats heard at $C$ is $18$. The speed of sound is $340 \, m/s$. The frequency of the source at $B$ is ..... $Hz$.

An engine is moving with uniform speed along a circular track emitting a sound of frequency $400\, Hz$ as shown in the figure. The speed of the engine is $30\, m/s$ and the speed of sound is $330\, m/s$. An observer is standing inside the track. The maximum frequency observed by the observer is