\(l=0.1 \,m\)
Fundamental frequency of pipe \(\left(f_1\right)=\frac{v}{4 l}\) or \(f_1=\frac{v}{0.4}\)
Frequency of \(2^{\text {nd }}\) overtone of open pipe \(2\) :
\(f_2=\frac{3 v}{2 l}\)
\(2 f_1=f_2\)
\(2 \times \frac{v}{0.4}=\frac{3 v}{2 l}\)
\(l=0.3\, m\)