A U-tube having uniform cross-section but unequal arm lengths $\ell_{1}$ and $\ell_{2}\left(<\ell_{1}\right)$ has same liquid of density $\rho_{1}$ filled in it up to a height $h$ as shown in Fig. $8.49$. Another liquid of density $\rho_{2}=\left(\rho_{1} / 2\right)$ is poured in $\operatorname{arm} A$. Both liquids are immiscible. What length of the second liquid should be poured in $A$ so that first overtone of $A$ is in unison with fundamental tone of B? (Take $\ell_{1}=5 \mathrm{~m}, \ell_{2}=1 \mathrm{~m}$ and $h=0.5 \mathrm{~m}$ )