Two charged concentric spher-
ical shells have radii $10.0 \mathrm{~cm}$ and $15.0 \mathrm{~cm}$. The charge on the inner shell is $4.00 \times 10^{-8} \mathrm{C}$, and that on the outer shell is $2.00 \times 10^{-8} \mathrm{C}$. Find the electric field at $r=12.0 \mathrm{~cm}$.
$2.50 \times 10^4 \mathrm{~N} / \mathrm{C}$