If the third harmonics of a closed organ pipe is equal to the fundamental frequency of an open organ pipe, compute the length of the open organ pipe (in cm) if the length of  the closed organ pipe is 30cm.

Let $l_2$  be the length of the open organ pipe, with $l_1=30cm$ the length of the closed oragan pipe.
It is given that the third harmonic of closed organ pipe is equal to the fundamental frequency of open organ pipe.
The third harmonic of a closed organ pipe is $f_2=\frac{v}{{\lambda }_2}=\frac{3v}{4l_1}=3f_1$
The fundamental frequency of open organ pipe is $f_1=\frac{v}{{\lambda }_1}=\frac{v}{2l_2}$
Therefore,

$\frac{v}{2l_2}=\frac{3v}{4l_1}\Rightarrow l_2=\frac{2l_1}{3}=20cm$