A strain gage located at $\backslash ($ C $\backslash )$ on the surface of bone $\backslash ($ A B $\backslash )$ indicates that the average normal stress in the bone is $3\mathrm{.}80$ MPa when the bone is subjected to two $1240-$N forces $\backslash ($ P $\backslash )$ as shown. Assuming the cross section of the bone at $\backslash ($ C $\backslash )$ to be annular and knowing that its outer diameter is $25$ mm $,$ determine the inner diameter of the bone's cross section at $\backslash ($ C $\backslash ).$

The inner diameter of the bone's cross section at $\backslash ($ C $\backslash )$ is

mm$.$