(1)

For closed organ pipe, fundamental frequency is given by  ${\upsilon }_c=\frac{v}{4l}$

For open organ pipe, fundamental frequency is given by ${\upsilon }_o=\frac{v}{2l\text{'}}$

$2^{nd}$ overtone of open organ pipe

           $\upsilon \text{'}=3{\upsilon }_o; \upsilon \text{'}=\frac{3v}{2l\text{'}}$

According to question, ${\upsilon }_c=\upsilon \text{'}$

        $\frac{v}{4l}=\frac{3v}{2l\text{'}} \Rightarrow l\text{'}=6l$

Here, $l=20\text{cm}, l\text{'}=?$

$\therefore$  $l\text{'}=6\times 20=120$ $\text{cm}$

(4)

Fundamental frequency of the closed organ pipe is $\upsilon =\frac{v}{4L}$

Here, $v=340$ $\text{m}$ $s^{-1}$, $L=85$ $\text{cm}=0.85$ $\mathrm{m}$

$\therefore$  $\upsilon =\frac{340 {\text{m s}}^{-1}}{4\times 0.85}=100$ Hz

The natural frequencies of the closed organ pipe will be

${\upsilon }_n=(2n-1)\upsilon =\upsilon ,3\upsilon ,5\upsilon ,7\upsilon ,9\upsilon ,11\upsilon ,13\upsilon ,\dots$

$=100 \text{Hz},300 \text{Hz},500 \text{Hz},700 \text{Hz},900 \text{Hz},1100 \text{Hz},1300 \text{Hz},\dots$

and so on

Thus, the number of natural frequencies lies below 1250 Hz is 6.