An open organ pipe and closed pipe have same length. The ratio of frequencies of their nth over tone is

Frequency of nth overtone for a open organ pipe

$\begin{array}{l}{f}_n=(n+1)\frac{C}{2l}\end{array}$

Frequency of nth overtone for a closed organ pipe

$f_n=(2n+1)\frac{C}{4l}$

Therefore, ratio of the frequencies is 

$(\mathrm{n}+1):\left(\frac{2\mathrm{n}+1}{2}\right)$