I'm having difficulty with a problem on MasteringPhysics (such wonderful software...) and as a last resort I'm posting on here. This is, I'm sure, a really simple problem but I'm getting no kind of feedback from MP and there isn't an example problem like this in the book. 1. The problem statement, all variables and given/known data Two aluminum spheres of mass .025 kg are separated by 80 centimeters. A) How many electrons does each sphere contain? B)How many electrons would have to be removed from one sphere and added to the other to cause an attractive force between the spheres of magnitude 1.00 x 10^4 (roughly one ton)? Assume that the spheres may be treated as point charges. C)What fraction of all the electrons in one of the spheres does this represent? 2. Relevant equations F=K*q1*q2/(r^2) q=e*(#protons-#electrons) 3. The attempt at a solution A) I found Part A to be 7.25 x 10^24 electrons. B) This is where I'm stuck. If you were to remove electrons from one sphere and put them on the other I understand that their charges are to be equal but opposite, as in q1 = -q2. So using Coloumb's Law (F=K*q1*q2/(r^2)) I've set the Force to 1*10^4, divided that by K=9*10^9. 10000/(9*10^9) = q1*q2/(.8^2) Then multiplying that by .8^2, I have just the charges on the other side of the equation. Since the charges in the formula are absolute value I can set q1=q2 and have q1^2. Taking the square root of the entire thing I have: q=8.4327*10^-4. So now I can use the formula q=e(#protons-#electrons). So: 8.4237*10^-4 = 1.6*10^-19(7.25*10^24-#electrons). Solving from this I get 7.249*10^24 electrons as my final answer. However, MP says I'm wrong but there isn't any feedback as to where I went wrong, and it seems straightforward enough to me that no matter how I rework it I get the same thing every time. Can anyone help me out here? C) Can't do this one until B is done. Any help is greatly appreciated.