Answer:
Force,

Step-by-step explanation:
Given that,
Charges on pith balls,

Distance between balls, d = 9 cm = 0.09 m
Let F is the repulsive force between two pith balls. We know that the repulsive force between two charges is given by :


F = 0.000871 N
or

So, the repulsive force between the pith balls is
. Hence, this is the required solution.