If the source and the observer are both relatively at rest and the speed of sound is increased,then the frequency heard by the observer will:

The engine of a train moving with speed $10\,ms^{-1}$ towards a platform sounds a whistle at frequency $400\,Hz$. The frequency heard by a passenger inside the train is $........\,Hz$ (neglect air speed. Speed of sound in air $330\,ms^{-1}$). (in $,Hz$)

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 $\%$)?

$A$ 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 $n$?

An engine sounding a whistle of frequency $2000 \,Hz$ is receding from a stationary observer at $72 \,km/h$. What is the apparent frequency heard by the observer (in $\,Hz$)? The velocity of sound in air is $340 \,m/s$.

$A$ train,standing at the outer signal of a railway station,blows a whistle of frequency $400\; Hz$ in still air.
$(i)$ What is the frequency of the whistle for a platform observer when the train $(a)$ approaches the platform with a speed of $10\; m s^{-1}$,$(b)$ recedes from the platform with a speed of $10\; m s^{-1}$?
$(ii)$ What is the speed of sound in each case? The speed of sound in still air can be taken as $340\; m s^{-1}$.