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 $v$)

$A$ 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 $n$?

Two passenger trains moving with a speed of $108\, km/h$ cross each other. One of them blows a whistle whose frequency is $750\, Hz$. If the speed of sound is $330\, m/s$,then the passengers sitting in the other train,after the trains cross each other,will hear a sound whose frequency is .... $Hz$.

When the observer moves towards a stationary source with velocity $V_{1}$,the apparent frequency of the emitted note is $F_{1}$. When the observer moves away from the source with velocity $V_{1}$,the apparent frequency is $F_{2}$. If $V$ is the velocity of sound in air and $F_{1} / F_{2} = 2$,then $V / V_{1}$ is equal to:

$A$ 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:

$A$ source of frequency $150 \, Hz$ is moving in the direction of a person with a velocity of $110 \, m/s$. The frequency heard by the person will be .... $Hz$ (speed of sound in medium $= 330 \, m/s$)