Question 397637
Let r = radius of inner ring.
Then {{{10^2pi - r^2pi = 90}}}
==> {{{100 - r^2 = 90/pi}}}
==> {{{r^2 = 100 - 90/pi}}}
==> {{{r = sqrt(100 - 90/pi) = 8.447}}}, to 3 decimal places.