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

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

If a source emitting waves of frequency $f$ moves towards an observer with a velocity $v/4$ and the observer moves away from the source with a velocity $v/6$,the apparent frequency as heard by the observer will be ($v =$ velocity of sound).

What should be the velocity of a sound source moving towards a stationary observer so that the apparent frequency is double the actual frequency? (Velocity of sound is $v$)

An observer moves towards a stationary source of sound of frequency $n$. The apparent frequency heard by him is $2n$. If the velocity of sound in air is $332 \, m/s$,then the velocity of the observer is .... $m/s$.

$A$ 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 $n$?