Physics 2 , two Styrofoam balls on insulated thread

1. The problem statement, all variables and given/known data
Two small .1 gram stryofoam balls are strung like beads on a vertical insulating thread. The lower ball is glued to the thread but the upper ball is free to move. Imagine that both are given an equal amount of negative charge such that the upper ball is suspended about the lower with their centers 4.0 cm apart. (Assume that each ball repels the other as if it were a point charge located at its center.) Roughly how many electrons have been added to each ball? Express your result as a fraction of the total number of electrons initially inside each ball.

2. Relevant equations

E = (kq1q2) / r^2

One electron = 1.6 x 10^-19 Coulombs

k = 9 X 10^ 9 N m^2 / c^2

3. The attempt at a solution

Assumed the force was now close to zero if the top ball is now suspended in the air instead of being pushed upward. But then you just get both charges to be zero which would be incorrect. Not sure any other way to do this.

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