The Coordinate Plane · Superchallenge problem

A rectangle has corners $A(0, 0)$, $B(8, 0)$, $C(8, 5)$ and $D(0, 5)$. A point $P$ is somewhere inside the rectangle, but you are not told where. What is the area of triangle $PAB$ plus the area of triangle $PCD$, in square units?