masses \(=\mathrm{m}_{1}=\mathrm{m}_{2}\)

density \(\left(\rho_{1} / \rho_{2}\right)=(1 / 3)\)

\(\mathrm{I}=\mathrm{Ml}\) of disc \(=\left[\left(\mathrm{MR}^{2}\right) / 2\right]\)

also \(\rho=[(\text { mass }) / \text { (volume) }]\)

\(=[(\text { mass }) /(\text { area } \times \text { thickness) }]\)

\(=\left[\mathrm{m} /\left(\pi \mathrm{R}^{2} \mathrm{t}\right)\right]\)

\(\mathrm{R}^{2}=[\mathrm{m} /(\pi \mathrm{t} \rho)]\)

from \(( 1)\)

\(\mathrm{I}=(\mathrm{M} / 2) \mathrm{R}^{2}\)

\(=(M / 2) \cdot[M /(\pi t \rho)]\)

\(=\left[M^{2} /(2 \pi T \rho)\right]\)

given: thickness \(\&\) masses same.

hence I \(\propto(1 / p)\)

\(\left(l_{1} / l_{2}\right)=\left(\rho_{2} / \rho_{1}\right)\) as \(t_{1}=t_{2} m_{1}=m_{2}\)

\(\left(l_{1} / l_{2}\right)=(3 / 1)\)