$$ \begin{aligned} & \mathrm{T}_{\text {air }}=2 \pi \sqrt{\frac{\ell}{\mathrm{g}}}=\sqrt{3} ~\mathrm{sec} \\ & \ln \text { Liquid } \rightarrow g_{\text {net }}=g\left(1-\frac{\rho}{\sigma}\right) \\ & \rho=\text { density of liquid } \\ & \sigma=\text { density of material of bob } \\ & \text { so } \mathrm{T}_{\text {Liq }}=2 \pi \sqrt{\left(\frac{\ell}{g_{\text {net }}}\right)}=2 \pi \sqrt{\frac{\ell}{g\left(1-\frac{\rho}{\sigma}\right)}} \\ & \mathrm{T}_{\text {Liq }}=\frac{\mathrm{T}_{\text {air }}}{\sqrt{1-\frac{\rho}{\sigma}}}=\frac{\sqrt{3}}{\sqrt{1-\frac{1}{4}}}=\frac{\sqrt{3}}{\frac{\sqrt{3}}{2}}=2 ~\mathrm{sec} \end{aligned} $$