Gauss's Law and charged conductors

[veevee]

A conductor carries a net charge +Q. There is a hollow cavity inside the conductor that contains a point charge -Q. In electrostatic equilibrium,what is the charge on the outer surface of the conductor?

A. -2Q
B. -Q
C. 0
D. Q
E. 2Q


I know that the answer is 0, however, I cannot understand why. i would think that the inner surface of the conductor is +Q to offset the -Q point charge inside, which would make the outer surface +Q in order to account for the net +Q charge. However, that is not the case.

Can some one please offer me some help understanding this problem?Thanks so much

 

[SammyS]

If the charge on the inner surface of the conductor is +Q, and the net charge on the conductor is also +Q, how much charge is left over to distribute on the outer surface?

 

[veevee]

Oh ok, I see. It's 0. For some reason i thought that the +Q and the -Q would cancel out.