Here's a link to the problem: http://grephysics.net/ans/0177/8

When there is a positive charge Q above an infinite grounded conducting plane, the method of images says the field is like that of the positive charge Q and negative charge -Q on the the opposite side of the plane. The problem here asks what the the total charge induced on the plane is, and the answer is -Q.

Can someone help me understand this? It isn't obvious to me why the charge distribution on the plane must have a total charge of -Q to produce a field above it that is as if there were a negative point charge -Q below it.

Thanks for the help!