$13-$ Two conducting spheres one with a radius of $0\mathrm{.}4$ m and a charge of $20$ nC and the other with a radius $0\mathrm{.}6$ m and a charge of $30$ nC are $16$ m apart center to center.

a$)$ find the electric potential at the mid point between them

b$)$ If the two spheres are connected using a very thin conductor, what is the charge on each