When a source of sound moves towards a stationary observer,the apparent frequency heard by him

$A$ source of sound is travelling with a velocity of $40 \, km/h$ towards an observer and emits sound of frequency $2000 \, Hz$. If the velocity of sound is $1220 \, km/h$,what is the apparent frequency heard by the observer in $Hz$?

$A$ bus moving with a uniform speed of $72 \,km/h$ towards a building blows a horn of frequency $1.7 \,kHz$. If the speed of sound in air is $340 \,m/s$, what will be the frequency of the echo heard by the bus driver (in $\,kHz$)?

$A$ source is stationary and the observer is in motion along a line joining the source and the observer. If the frequency heard by the observer is $1\%$ higher than the true frequency,the ratio of velocity of the observer and that of sound in air is:

$A$ motorcyclist is moving around a circular track of radius $50 \ m$ with a constant speed of $25 \ m/s$. $A$ static siren at point $Y$ emits sound of frequency $n$. How many times (approximately) in an hour will the motorcyclist hear the sound of the actual frequency $n$?

$A$ source of sound emits sound waves at frequency $f_0$. It is moving towards an observer with fixed speed $v_s$ ($v_s < v$,where $v$ is the speed of sound in air). If the observer were to move towards the source with speed $v_0$,the variation of the frequency $f$ heard by the observer as $v_0$ is changed is given by one of the following two graphs ($A$ and $B$). The variation of $f$ with $v_0$ is given correctly by: