Doppler's effect will not be applicable when the velocity of sound source is

$A$ car $P$ approaching a crossing at a speed of $10 \, m/s$ sounds a horn of frequency $700 \, Hz$ when $40 \, m$ in front of the crossing. The speed of sound in air is $340 \, m/s$. Another car $Q$ is at rest on a road which is perpendicular to the road on which car $P$ is reaching the crossing. The driver of car $Q$ hears the sound of the horn of car $P$ when he is $30 \, m$ in front of the crossing. The apparent frequency heard by the driver of car $Q$ is ..... $Hz$.

$A$ musician on a moving vehicle plays a tone at $880 \ Hz$. When the vehicle is approaching a listener, the listener receives it as an $888 \ Hz$ tone. What is the speed of the vehicle (in $m \ s^{-1}$)? (Assume the velocity of sound is $333 \ m \ s^{-1}$)

An observer is riding on a bicycle and moving towards a hill at $18\,km\,h^{-1}$. He hears a sound from a source at some distance behind him directly as well as after its reflection from the hill. If the original frequency of the sound as emitted by the source is $640\,Hz$ and the velocity of sound in air is $320\,m/s$,the beat frequency between the two sounds heard by the observer will be $...Hz$.

$A$ source with frequency $f$ and an observer are moving towards each other with a velocity of $V/10$. The frequency heard by the observer will be ............ $f$. (The velocity of sound in air is $V \, m/s$.)

$A$ whistle revolves in a circle with an angular speed of $20 \; rad/s$ using a string of length $50 \; cm.$ If the frequency of sound from the whistle is $385 \; Hz,$ then what is the minimum frequency heard by an observer,who is far away from the centre in the same plane? $(v = 340 \; m/s)$