At what speed should a source of sound move so that the observer finds the apparent frequency equal to half the original frequency?

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 speed of sound in air and $F_1/F_2 = 2$, then $V/V_1 =$

With what velocity should an observer approach a stationary sound source,so that the apparent frequency of sound appears double the actual frequency? (Let $v$ be the velocity of sound).

Two vehicles,each moving with speed $u$ on the same horizontal straight road,are approaching each other. Wind blows along the road with velocity $w$. One of these vehicles blows a whistle of frequency $f_1$. An observer in the other vehicle hears the frequency of the whistle to be $f_2$. The speed of sound in still air is $V$. The correct statement$(s)$ is (are) :
$(A)$ If the wind blows from the observer to the source,$f_2 > f_1$.
$(B)$ If the wind blows from the source to the observer,$f_2 > f_1$.
$(C)$ If the wind blows from the observer to the source,$f_2 < f_1$.
$(D)$ If the wind blows from the source to the observer,$f_2 < f_1$.

$A$ policeman blows a whistle of frequency $400 \ Hz$. $A$ car driver is approaching the policeman. The speed of the car is $54 \ km/h$. The change in frequency experienced by the driver,when the driver approaches the policeman and after he crosses the policeman,is ... $Hz$ [Velocity of sound is $350 \ m/s$]

An obstacle is moving towards the source with velocity $v$. The sound is reflected from the obstacle. If $c$ is the speed of sound and $\lambda$ is the wavelength,then the wavelength of the reflected wave $(\lambda_{r})$ is