$3\mathrm{.}9$

Consider an insulating sphere with $10C$ of charge uniformly distributed through its volume. The sphere is surrounded by a conducting spherical shell that has a total charge of $-3C.$ Outside the conucting shell is an insulating shell with total charge $20C$ uniformly distributed through its volume. In the figure below, which shows a cross section of the structure, the darker shaded regions are insulators and the lighter shaded region is the conductor.