$\text { (Third harmonic) }=3 \times v_{0}^{\prime}=3 \times \frac{v}{2 L^{\prime}}$
First overtone of a closed organ pipe
$\text { (Third harmonic) }=3 \times v_{0}=3 \times \frac{v}{4 L}$
According to question,
$3 v_{0}^{\prime}=3 v_{0} \Rightarrow 3 \times \frac{v}{2 L^{\prime}}=3 \times \frac{v}{4 L}$
$\therefore \quad L^{\prime}=2 L$
The velocity of the wave is $.....ms ^{-1}$ [all the quantities are in SI unit]