By conservation of angular momentum $\begin{aligned}
& \mathrm{I}_1 \omega_1=\mathrm{I}_2 \omega_2 \
& \left(\frac{2}{5} \mathrm{MR}^2\right) \frac{2 \pi}{\mathrm{T}_1}=\frac{2}{5} \mathrm{M}\left(\frac{3}{4} \mathrm{R}\right)^2 \frac{2 \pi}{\mathrm{T}_2} \
& \frac{1}{\mathrm{T}_1}=\frac{9}{16 \mathrm{T}_2} \
& \frac{1}{\mathrm{~T}_2}=\frac{9}{16} \times \mathrm{T}_1=\frac{9}{16} \times 24 \mathrm{hr}=\frac{27}{2} \mathrm{hr}=13 \mathrm{hr} 30 \mathrm{mins} .
\end{aligned}$