When the source of sound moves towards a stationary observer,the wavelength of sound received by him

$A$ car sounding a horn of frequency $1000 \ Hz$ passes a stationary observer. The ratio of the frequencies of the horn noted by the observer before and after the passing of the car is $11 : 9$. If the speed of sound is $v$,what is the speed of the car?

$A$ stationary observer receives sound from two identical tuning forks, one of which approaches and the other one recedes with the same speed $v$ (much less than the speed of sound). The observer hears $2 \; \text{beats/sec}$. The oscillation frequency of each tuning fork is $\nu_{0} = 1400 \; \text{Hz}$ and the velocity of sound in air is $c = 350 \; \text{m/s}$. The speed of each tuning fork is close to:

$A$ 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 $n$?

$A$ motorcycle starts from rest and accelerates along a straight path at $2 \; m/s^2$. At the starting point of the motorcycle,there is a stationary electric siren. How far has the motorcycle gone when the driver hears the frequency of the siren at $94 \%$ of its original value (in $; m$)? (Speed of sound $= 330 \; m/s$)

$A$ man is standing on a railway platform listening to the whistle of an engine that passes the man at a constant speed without stopping. If the engine passes the man at time $t_0$,how does the frequency $f$ of the whistle as heard by the man change with time?