Two congruent squares, ABCD and PQRS, have side length 15. They overlap to form the 15 by 25 rectangle AQRD shown. What percent of the area of rectangle AQRD is shaded?[asy]
filldraw((0,0)--(25,0)--(25,15)--(0,15)--cycle,white,black);
label("D",(0,0),S);
label("R",(25,0),S);
label("Q",(25,15),N);
label("A",(0,15),N);
filldraw((10,0)--(15,0)--(15,15)--(10,15)--cycle,mediumgrey,black);
label("S",(10,0),S);
label("C",(15,0),S);
label("B",(15,15),N);
label("P",(10,15),N);
[/asy]<spanclass=′latex−bold′>(A)</span>15<spanclass=′latex−bold′>(B)</span>18<spanclass=′latex−bold′>(C)</span>20<spanclass=′latex−bold′>(D)</span>24<spanclass=′latex−bold′>(E)</span>25