Given
4th armonic 31.5Hz
Procedure
4f = 31.5
[tex]\begin{gathered} 4f=31.5 \\ f=\frac{31.5}{4} \\ f=7.875\text{ Hz} \end{gathered}[/tex]Thrid armonic
[tex]\begin{gathered} 3f=3\cdot7.875 \\ 3f=23.625\text{ Hz} \end{gathered}[/tex]Hence the third frequency is 23.625 Hz
