Which order of harmonics is missing or absent in case of stationary sound waves produced in a closed pipe?

Two closed organ pipes having lengths $20\,cm$ and $20.5\,cm$ produce $5\,beats/sec$. Determine the frequency of both organ pipes.

$A$ tuning fork is used to produce resonance in a glass tube. The length of the air column in this tube can be adjusted by a variable piston. At room temperature of $27\,^{\circ}C$,two successive resonances are produced at $20\,cm$ and $73\,cm$ of column length. If the frequency of the tuning fork is $320\,Hz$,the velocity of sound in air at $27\,^{\circ}C$ is .... $m/s$.

The second overtone of an open organ pipe is equal to the first overtone of a closed organ pipe. If the length of the closed organ pipe is $15 \ cm$,the length of the open organ pipe is: (in $cm$)

The frequency of the fourth overtone of a closed pipe is in unison with the fifth overtone of an open pipe. The ratio of the length of the closed pipe to that of the open pipe is

In a closed-end pipe of length $105 \, cm$,standing waves are set up corresponding to the third overtone. What distance from the closed end,amongst the following,is a pressure node in $cm$?