At the corners A, B, C of a square ABCD, charges 10 mC, –20 mC and 10 mC are placed. The electric intensity at the centre of the square to become zero, the charge to be placed at the corner D is:

The electric field at the point O due to charges at A and C will be canceled out as the electric field due to these are equal in magnitude and opposite in directions.

For zero electric field at the point O, the electric field due to charges at B and D should be equal in magnitude and opposite in direction. This is possible when charges will be same at both the points.