$A$ source of sound and a listener are approaching each other with a speed of $40 \, m/s$. The apparent frequency of the note produced by the source is $400 \, cps$. Then,its true frequency (in $cps$) is (velocity of sound in air $= 360 \, m/s$).

The speed of sound in air at a given temperature is $350 \, m/s$. An engine blows a whistle at a frequency of $1200 \, cps$. It is approaching the observer with a velocity of $50 \, m/s$. The apparent frequency in $cps$ heard by the observer will be:

$A$ car is moving at a velocity of $17 \ m/s$ towards an approaching bus that blows a horn at a frequency of $640 \ Hz$ on a straight track. The frequency of this horn appears to be $680 \ Hz$ to the car driver. If the velocity of sound in air is $340 \ m/s$,then the velocity of the approaching bus is: (in $m/s$)

$A$ train $S_1$,moving with a uniform velocity of $108 \ km/h$,approaches another train $S_2$ standing on a platform. An observer $O$ moves with a uniform velocity of $36 \ km/h$ towards $S_2$,as shown in the figure. Both the trains are blowing whistles of the same frequency $120 \ Hz$. When $O$ is $600 \ m$ away from $S_2$ and the distance between $S_1$ and $S_2$ is $800 \ m$,the number of beats heard by $O$ is: [Speed of sound $= 330 \ m/s$]

$A$ person observes two moving trains. The first reaches the station and the second leaves the station,both with an equal speed of $30 \ m/s$. If both trains emit sounds of frequency $300 \ Hz$,what is the difference in frequencies heard by the person (in $Hz$)? (Speed of sound in air $= 330 \ m/s$)

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: