Solving Spherical Capacitors: Find C1 & C2 in Parallel

[unscientific]

Homework Statement

Homework Equations

The Attempt at a Solution

Working backwards I found that adding C₁ (of radius a and b) and C₂ (of radius b and c) in parallel gives the answer. Not sure why they can be modeled as capacitors in parallel though..

 

[ehild]

Consider the inner sphere and the inner surface of the middle shell as one capacitor, (blue) and the outer surface of the middle shell and the outer shell as the other one (yellow). A and G are the terminals of the resultant capacitor.
How are the capacitors connected? ehild