\(I=\frac{1}{2} V \rho r^2\)
\(I=\frac{1}{2} \pi r^2 t \rho r^2\)
\(I=\frac{\pi r^4 t \rho}{2}\)
\(\pi r_1^2 t \rho_1=\pi r_2^2 t \rho_2\)
\(r_1^4 \rho_1^2=r_2^4 \rho_2^2\)
\(\frac{r_1^4}{r_2^4}=\frac{\rho_2^2}{\rho_1^2}\)
So, \(\frac{I_1}{I_2}=\frac{\pi r_1^4 t \rho_1}{\pi r_2^4 t \rho_2}=\frac{\rho_2^2}{\rho_1^2} \cdot \frac{\rho_1}{\rho_2}=\frac{\rho_2}{\rho_1}=3\)