The fundamental frequency in an open organ pipe is equal to the third harmonic of closed organ pipe. If the…

For closed organ pipe, third harmonic $\frac{3v}{4l_{\mathrm{cop}}}$
For open organ pipe, fundamental frequency $\frac{v}{2l_{\mathrm{oop}}}$.

According to question
$f_{\mathrm{o}\mathrm{o}\mathrm{p}}=3f_{\mathrm{c}\mathrm{o}\mathrm{p}}$
$\Rightarrow \frac{v}{2l_{\mathrm{o}\mathrm{o}\mathrm{p}}}=\frac{3v}{4l_{\mathrm{c}\mathrm{o}\mathrm{p}}}$
$l_{\mathrm{o}\mathrm{o}\mathrm{p}}=\frac{2}{3}\times l_{\mathrm{c}\mathrm{o}\mathrm{p}}=\frac{2}{3}\times 20\approx 13.2 \text{c}\text{m}$