Moment of Tnertia of a disk \((I)=\frac{1}{2} m k^2=\frac{1}{2} m\left(\frac{d}{2}\right)^2\) ratio of moment of inertia:

\(\frac{I_1}{I_2} =\frac{1}{2} m_1\left(\frac{d_1}{2}\right)^2 / 1 / 2 m_2\left(\frac{d_2}{2}\right)^2\)

\(=\frac{m_1 d_1^2}{m_2 d_2^2}=\frac{m_1\left(2 d_2\right)^2}{\left(2 m_1\right) d_2^2}\)

\(\Rightarrow \frac{I_1}{I_2} =\frac{(2)^2}{2}=\frac{4}{2}=2\)

Hence \(I_1: I_2=2: 1\)