The electric potential immediately outside a charged conducting sphere is 200 V, and 10.0 cm farther from the center of the sphere the potential is 150 V. Determine (a) the radius of the sphere and (b) the charge on it. The electric potential immediately outside another charged conducting sphere is 210 V, and 10.0 cm farther from the center the magnitude of the electric field is 400 V/m. Determine (c) the radius of the sphere and (d) its charge on it. (e) Are the answers to parts (c) and (d) unique?

for the first part I said 200=kq/r and 150 =kq/(r+0.1) to solve for r=0.3m I then input this in and solved for q=6.67×10^-9C

For the second part I said 210=kq/r and 400=q/(4pi(r+0.1)^2) and get r=-0.1 which cannot be right! help would be appreciated thanks

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