A  ball with charge $-$ 50 e is placed at the centre of a hollow spherical shell having a charge of $-$ 50 e. What is the charge on the shell's outer surface?

Let the induced charge on inner surface and outer surface is q₁ and q₂ respectively, 

then q₁ + q₂ = $-50 e$.

Here, charge q₁ induced due to ball is + 50e.

So, + 50e + q₂ = $-50 e$

$\Rightarrow q_2 = -100e$