When bob is inserted, in liquid effective of is reduced because of the force of upthrust.

$\rho_s$ is density of solid.

$\rho_L$ is density of liquid.

$V$ is volume of solid

and $T=2 \pi \sqrt{\frac{1}{g}}$

$T_{\text {new }}=2 \pi \sqrt{\frac{1}{g_{\text {new }}}}$

$T_{\text {new }}=2 \pi \sqrt{\frac{1}{g-\frac{\rho_L}{\rho_S} g}}$

$T_{\text {new }}=2 \pi \sqrt{\frac{1 \times 12}{g \times 11}}$

or $T_{\text {new }}=\sqrt{\frac{12}{11}} T$