Homework 23.45

Two charged concentric spherical shells have radii 10.0 cm and 15.0 cm. The charge on the inner shell is 4.00 x 10^-8 C, and that on the outer shell is 2 x 10^-8 C. Find the electric field (a) at r = 12.0 cm and (b) at r = 20.0 cm.