The diagram below shows a rectangle and two triangles with areas 20 and 4. What is the area of the shaded triangle?[asy]
size(3cm);
draw((0,0)--(7,0)--(7,5)--(0,5)--(0,0));
draw((4,0)--(0,5));
draw((7,0)--(0,5));
draw((4,0)--(7,5));
filldraw((0,0)--(4,0)--(0,5)--cycle, lightgray);
label("4",(5.5,0.5));
label("20",(4,4));
[/asy]
<spanclass=′latex−bold′>(A)</span>12<spanclass=′latex−bold′>(B)</span>14<spanclass=′latex−bold′>(C)</span>16<spanclass=′latex−bold′>(D)</span>18<spanclass=′latex−bold′>(E)</span>20