A spherical shell with an inner radius 'a' and an outer radius 'b' is made of conducting material. A point charge +Q is placed at the center of the spherical shell and a total charge −q  is placed on the outer surface of the shell. What is the total charge on the surface?

−Q on the inner surface and  −q+Q on the outer surface

Charge placed at the centre of a shell is +Q. Hence, a charge of magnitude - Q will be induced to the inner surface of the shell. Therefore, total charge on the inner surface of the shell is - Q.

A charge of +Q is induced on the outer surface of the shell. A charge of magnitude -q is placed on the outer surface of the shell. Therefore, total charge on the outer surface of the shell is Q - q