Normal time period $T=2 \pi \sqrt{\frac{l}{g}}$
When immersed in a liquid. It experiences an upthrust.
Upthrust $=\frac{\rho}{4} \times$ volume $g$
Upward acceleration $=$ Upward force $/$ mass of ball $=\frac{g}{4}$
$T^{\prime}=2 \pi \sqrt{\frac{I}{g_{e f f}}}$
$g_{\text {eff }}=g-\frac{g}{4}=\frac{3}{4} g$
$T^{\prime}=2 \pi=\sqrt{\frac{1}{3 g} \times 4}=\frac{2 T}{\sqrt{3}}$


