1. The problem statement, all variables and given/known data PQRS is a square. Three point charges -Q, +Q and -3Q are fixed at P, Q and S respectively as shown. A point charge q is then placed at R and it experiences a net electric force towards S. Which of the following is equal to q? photo : http://s613.photobucket.com/user/Yan_Wa_Chung/media/01_zps421rebzc.png.html 2. Relevant equations F=kq1q2/r^2 3. The attempt at a solution in any kind of calculation, Q and q are cancelled out and undetermined ,ie : (k1) (q) (Q) / (r1)^2 = (k2) (q) (Q) / (r2)^2, (turn force PR to vertical and horizontal component cancel the vertical component with force QR) . I have no idea on the question or maybe the question itself has mistakes ?The right answer is q=1/9 Q.