A charge g is placed at the centre of the line joining the equal charges Q. The system of three charges will be in equilibrium if q is equal to

Let the distance between the two charges Q and Q be r. Force on Q due to charge g at a distance (r /2).=14πε0×qQ(r/2)2Force on Q due to other charge Q at a distance r=14πε0×Q×Q(r)2∴ 14πε0×qQ(r/2)2+14πε0Q×Qr2=04q=−Q or q=−Q/4