Answer
The electric potential at the center of the sphere is $~~400~V$
Work Step by Step
The electric field inside a hollow sphere is zero.
Therefore, the sphere must have the same electric potential at all points on the surface and within the sphere.
Note that if there were two points within a sphere with a different electric potential, then there would be an electric field between the two points (which is a contradiction).
Therefore, the electric potential at the center of the sphere is $~~400~V$
