Length of the organ pipe is same in both the cases. Fundamental frequency of open pipe is ${\text{f}}_1=\frac{\text{v}}{2\mathit{\text{l}}}$ Frequency of third harmonic of closed pipe will be.

                  $f_2=3\left(\frac{v}{4l}\right)$

Given that, $f_2=f_1+100$

or               $f_2-f_1=100$

or               $\frac{3}{4}\left(\frac{v}{l}\right)-\left(\frac{1}{2}\right) \left(\frac{v}{l}\right)=100\Rightarrow \frac{v}{4l}=100$

$\therefore$               $\frac{v}{2l}\text{ or }{f}_1=200\text{ Hz}$ $\text{50}\text{.}\text{n}\text{Hz}\Rightarrow \text{n = 4}$

Therefore, fundamental frequency of the open pipe is 200 Hz. and $\text{n = 4}$