CLOSED organ pipe \[{{n}_{1}}=\frac{3V}{4{{l}_{1}}}\] Third overtone of open organ pipe \[{{n}_{2}}=\frac{4V}{2{{L}_{2}}}\] \[{{n}_{1}}={{n}_{2}}\] (GIVEN) \[\Rightarrow \]\[\frac{3v}{4{{L}_{1}}}=\frac{4v}{2{{L}_{2}}}\Rightarrow \]\[\frac{{{L}_{1}}}{{{L}_{2}}}=\frac{3}{8}\]