A conducting sphere with a radius of 20 cm was surrounded by a thin spherical shell with a radius of 40 cm

A conducting sphere with a radius of 20 cm was surrounded by a thin spherical shell with a radius of 40 cm and a charge of 2 μC. The sphere was grounded. Determine the sphere charge and the shell potential after grounding.

The problem is about a spherical capacitor, the capacity of which is found by the following formula:
С = 9 * 10⁻⁹ * R1 * R2 / (R1 + R2);
Let’s count:
С = 9 * 20 * 40 / (20 + 40) * 10⁻⁹;
We get:
C = 120 * 10⁻⁹ F.
Now let’s find the potential:
φ = q / C;
φ = 2 * 10⁻⁶ / 120 * 10⁻⁹;
φ ≈ 17 V.
Answer: φ ≈ 17 V.