Three identical metal plates with large surface area are kept plate parallel to each other as shown in fig. The leftmost plate is given charge Q, the rightmost given charge $-2Q$  and the middle one remains neutral. Then the charge appearing on outer surface of the rightmost plate is
 

Distribution of charge on different surface plates has been shown

Since inside a conductor, E = 0

Take a point P on the leftmost plate. The electric field is

$E=\frac{Q-q}{2A{\epsilon }_0}-\frac{q}{2A{\epsilon }_0}+\frac{q}{2A{\epsilon }_0}-\frac{q}{2A{\epsilon }_0}+\frac{q}{2A{\epsilon }_0}-\frac{(q-2Q)}{2A{\epsilon }_0}=0$ 
 

So,   $Q-q-q+2Q=0$
or  $2q=3Q\Rightarrow q=\frac{3Q}{2}$
The charge appearing on the outer surface of the rightmost plate is
$q-2Q=\frac{3Q}{2}-2Q=-\frac{Q}{2}$