$7\mathrm{.}2\mathrm{.}35.[$Level $3]$ The body illustrated is a simplified model of a bicycle's chain ring. Determine its mass moment of inertia $($in$-$plane rotation$)$ about $O.=8\mathrm{.}6{10}^{-4}k\frac{g}{c}{m}^2,a=2\mathrm{.}5cm.$