Hello!!!!

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Given that a charge q is placed at the center C of line joining two equal charges Q each,at points A and B.

Let AB=2a

If the net force on each charge will be 0(zero) ,then the system will be in equilibrium.

let the charge Q is present at A.

At equilibrium state ,the total force on this charge is given by

1/4π e0*qQ/a^2 + 1/4π e0*qQ/2a^2 =0

=1/4π e0*qQ/a^2 = - 1/4π e0*qQ/2a^2

on solving we get,

q=-Q/4

Hope it helps!!!!

Thank you!!!!