An electricity and magnetism problem by Jonathan Schirmer

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 $$a+b$$?

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