Answer
The charge on the outer surface of the conductor is $~~+0.80\times 10^{-9}~C$
Work Step by Step
In part (a), we found that the charge on the conductor is $~~+2.0\times 10^{-9}~C$
In part (c), we found that the charge on the inner surface of the conductor is $~~+1.2\times 10^{-9}~C$
Since the electric field within a conductor is zero, the charge is on the inner surface or the outer surface of the conductor.
Therefore, the charge on the outer surface of the conductor must be $~~+0.80\times 10^{-9}~C$
