\({{\text{I}}_{\text{1}}}{\omega _1}\, = \,\,{I_2}{\omega _2}\,\,\therefore \,\,\frac{2}{5}\,\,M{R^2}\,\,\frac{{2\pi }}{{{T_1}}}\,\,\)
\(= \,\,\frac{2}{5}\,\,M{\left( {\frac{R}{n}} \right)^2}\,\,\frac{{2\pi }}{{{T_2}}}\,\, = \,\,{T_2}\,\, = \,\,\frac{{{T_1}}}{{{n^2}}}\)