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

Two trains move towards each other with the same speed. The speed of sound is $340 \; m/s$. If the pitch of the whistle of one train heard by an observer on the other train changes by a factor of $9/8$,then the speed of each train is ........... $m/s$.

Two trains $A$ and $B$ are moving towards each other with speeds $72 \ km/h$ and $36 \ km/h$ respectively. The train $A$ whistles at $640 \ Hz$ frequency. Before the trains meet,the frequency of sound heard by a passenger in Train $B$ is (Speed of sound in air $= 340 \ m/s$). (in $Hz$)

Two trucks heading in opposite directions, each with a speed of $0.1 u$, approach each other. The speed of sound is $u$. The driver of the first truck sounds his horn with a frequency of $495 \,Hz$. Let $v_1$ and $v_2$ be the frequencies heard by the driver of the second truck when the trucks are approaching each other and when the trucks have passed each other, respectively. The magnitude of $v_1 - v_2$ is (in $\,Hz$)

Obtain the equation for the frequency observed by a stationary observer when the source is moving.

$A$ source of sound $S$ of frequency $500 \ Hz$ situated between a stationary observer $O$ and a wall $W$,moves towards the wall with a speed of $2 \ m/s$. If the velocity of sound is $332 \ m/s$,then the number of beats per second heard by the observer is (approximately)