$A$ set of $20$ tuning forks is arranged in a series of increasing frequencies. If each fork gives $4 \; Hz$ beats with respect to the preceding fork and the frequency of the last fork is twice the frequency of the first,then the frequency of the last fork is $\dots \; Hz$.

The disc of a siren containing $60$ holes rotates at a constant speed of $360\,rpm$. The emitted sound is in unison with a tuning fork of frequency .... $Hz$

The sound waves of wavelengths $5 \,m$ and $6 \,m$ produce $30$ beats in $3 \,s$. The velocity of sound is (in $\,m/s$)

Two sitar strings $A$ and $B$ playing the note 'Dhai' are slightly out of tune and produce beats of frequency $5 \; Hz$. The tension of the string $B$ is slightly increased and the beat frequency is found to decrease to $3 \; Hz$. What is the original frequency of $B$ if the frequency of $A$ is $427 \; Hz$ (in $; Hz$)?

Two tuning forks $A$ and $B$ are sounded together and produce $6$ beats per second. The frequency of tuning fork $B$ is $384 \,Hz$. When tuning fork $A$ is loaded with wax,the number of beats per second becomes $4$. What is the frequency of tuning fork $A$ in $Hz$?

Two tuning forks of frequencies $320 \ Hz$ and $323 \ Hz$ are vibrated together. The time interval between a maximum sound and its adjacent minimum sound heard by an observer is