A solid metal sphere of radius $R$ has charge $+2Q$. A hollow spherical shell of radius $3R$, concentric with the first sphere, has net charge $-Q$.
What would be the final distribution of the charge if the spheres were joined by a conducting wire?
Answer: $+Q$ on outer shell, $0$ on inner sphere.
Why is that? I would assume that the total charge should just evenly spread between the spheres.