two charges +Q each are placed 'd' distance apart where should be the third charge q be placed so that all the charges are in equilibrium? what is the sign and magnitude of q

If 2 charges of size Q are placed at a d distance form  one another and we need to place q, third charge such that all the charges are in equilibrium, then it is impossible. 

 

If we want the charge q to be in equilibrium, it needs to be placed on the axis joining the 2 Q charges. And furthermore, to be in equilibrium, q needs to be placed at a distance = d/2 from one of the charge.  But is it is placed at d/2 from one of the charges, then neither of the 2 Q charges will be in equilibrium.