$A$ man is watching two trains,one leaving and the other coming with an equal speed of $4\,m/s$. If they sound their whistles each of frequency $240\,Hz$,the number of beats per second heard by the man will be equal to: (velocity of sound in air $= 320\,m/s$)

$A$ 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?

$A$ 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 ..... $Hz$.

$A$ submarine $(A)$ travelling at $18\, km/hr$ is being chased along the line of its velocity by another submarine $(B)$ travelling at $27\, km/hr$. $B$ sends a sonar signal of $500\, Hz$ to detect $A$ and receives a reflected sound of frequency $v$. The value of $v$ is close to ... $Hz$ (Speed of sound in water $= 1500\, ms^{-1}$)

An observer moves towards a stationary source of sound with a velocity of one-fifth of the velocity of sound. The percentage increase in the apparent frequency is (in $\%$)

Two cars $A$ and $B$ are moving in the same direction with speeds $36 \, km/hr$ and $54 \, km/hr$ respectively. Car $B$ is ahead of $A$. If $A$ sounds a horn of frequency $1000 \, Hz$ and the speed of sound in air is $340 \, m/s$,the frequency of sound received by the driver of car $B$ is ................. $Hz$.