Physics
posted by Mari .
A uniformly charged sphere has a potential on its surface of 410 V. At a radial distance of 20 cm from this surface, the potential is 150 V. The Coulomb constant is 8.99 × 109 N · m2/C2.What is the radius of the sphere? Answer in units of m.

The potential is proportional to 1/R, where R is the distance from the center. You don't need Coulomb's constant for this one. Let R1 be the surface and R2 = 20 cm.
V2/V1 = R1/R2
150/410 = R1/20
R1 = 7.37 cm