Quadrilateral ABCD is a trapezoid, AD=15, AB=50, BC=20, and the altitude is 12. What is the area of the trapezoid?[asy]
pair A,B,C,D;
A=(3,20);
B=(35,20);
C=(47,0);
D=(0,0);
draw(A--B--C--D--cycle);
dot((0,0));
dot((3,20));
dot((35,20));
dot((47,0));
label("A",A,N);
label("B",B,N);
label("C",C,S);
label("D",D,S);
draw((19,20)--(19,0));
dot((19,20));
dot((19,0));
draw((19,3)--(22,3)--(22,0));
label("12",(21,10),E);
label("50",(19,22),N);
label("15",(1,10),W);
label("20",(41,12),E);[/asy]<spanclass=′latex−bold′>(A)</span>600<spanclass=′latex−bold′>(B)</span>650<spanclass=′latex−bold′>(C)</span>700<spanclass=′latex−bold′>(D)</span>750<spanclass=′latex−bold′>(E)</span>800 geometrytrapezoidPythagorean Theorem