An electricity and magnetism problem by Jonathan Schirmer

Electricity and Magnetism Level 4

Consider a cube made of six conducting plates. One is held at a potential of $20$ $V$ and the other five are grounded. The potential at the center of the cube can be expressed in Volts as $\frac{a}{b}$ where $a$ and $b$ are coprime positive integers. What is the value of $a+b$ ?