Question

What minimum work must be done by an external force to bring a charge q = 3.00 μC from a great distance away (take r = ∞) to a point 0.500 m from a charge Q = 20.0 µC?

Answer 1

I think that your work W, is equals to the potential energy of the new system composed by the two charges, namely:
\[W=\frac{ 3*10^{-6} *20*10^{-6}}{ 5*10^{-1} }=1.2*10^{-10}Joule\]