We know from Coulomb's law that electrostatic force between two charges is given by -
F = K * q1 * q2 / r^2
K is a constant and r is the distance between two charges.
Now, in the given question
q2 will be static if the force exert by q1 on q2 is equal and opposite to the force exert by Q on q2
K* q1*q2 / 20^2 = - K*Q*q2 / 10^2 (negative sign is to show opposite forces)
q1/400 = -Q / 100
q1 = -4Q
q1 = -4 * (-7.5nC)
q1 = 30 nC
Let me know if you need further help.
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