Examining the diagram, we can see that B is equidistant from A and C. According to the Converse of the Perpendicular Bisector Theorem, if a point is equidistant from the endpoints of the segment, then it lies on its perpendicular bisector. This means that BD is a perpendicular bisector of AC.
Since the perpendicular bisector passes through the midpoint of the segment, we know that AD=DC.
We are given that AD=20. Therefore, DC=20 as well.
