b
$g^{\prime}=\frac{m g-F_{B}}{m}$
$=\frac{\rho_{B} V g-\rho_{\pi} V g}{\rho_{B} V}$
$=\left(\frac{\rho_{B}-\rho_{\pi}}{\rho_{B}}\right) g \quad T=2 \pi \sqrt{\frac{\ell}{g}}$
$=\frac{5-1}{5} \times g$
$=\frac{4}{5} g$
$\frac{T^{\prime}}{T^{\prime}}=\sqrt{\frac{g}{g^{\prime}}}=\sqrt{\frac{g}{5} g}=\sqrt{\frac{5}{4}}$
$T^{\prime}=T \sqrt{\frac{5}{4}}=\frac{10}{2} \sqrt{5}$
$T^{\prime}=5 \sqrt{5}$
