fundamental frequency

       


$$\begin{gathered} \text{L}=\frac{\lambda }{4} \\ {v}_1=\frac{\text{v}}{4\text{L}} \end{gathered}$$


       


$\text{L}=\frac{\lambda }{2}+\frac{\lambda }{4}=\frac{3\lambda }{4}$

$v_2=\frac{3\text{v}}{4\text{L}}$

$\therefore v=\text{odd}\left(\frac{\text{v}}{4\text{L}}\right)$

Now $\text{L}=85cm=\text{0.85 m}$

        $\text{v}=340m/s$

         $=\text{odd}\left(\frac{340}{4\times \text{0.85}}\right)$

         $=\text{odd}\times 100 \text{Hz}$

Hence possible frequencies below 1250 Hz are

100 Hz, 300 Hz, 500 Hz...........1100 Hz

Hence 6