Equipotential surfaces; finding the radius

[IHave]

Homework Statement

a metal sphere of radius 0.39 m carries a charge 0.55 μC. Equipotential surfaces are to be drawn for 100-V intervals outside of the sphere.

Determine the radius of the first, tenth and 100th equipotential from the surface.

Homework Equations

V = kQ / r
Volt = (Coulombs Constant * Charge) / radius

The Attempt at a Solution

r = ((9.0X10^9 Nm²/C²)(.55X10^-6 C))/ (1 Nm/C)

r = ((9.0X10^9 Nm²/C²)(.55X10^-6 C))/ (10 Nm/C)

r = ((9.0X10^9 Nm²/C²)(.55X10^-6 C))/ (100 Nm/C)

all units cancel out and I'm left with meters but I'm getting the incorrect values according to the homework system that I'm using

 

[SammyS]

Read the problem more carefully. The equipotential surfaces are 100 V apart.

What is the potential of the sphere itself? Start there.