If you don't use algebra, you can think like this:

(1) Look at the two rectangles on the left first. They have the same side, so the ratio of the other side is equal to its area ratio. In other words, the width of the shaded part is 20/8=2.5 times that of the rectangle in the upper left corner.

(2) Look at the two rectangles above. They also have the same side, so the ratio of the other side is equal to its area ratio. That is to say, the length of the shaded part is 16/8=2 times that of the upper left rectangle.

(3) So the area of the shadow rectangle is 2×2.5 times that of the upper left corner rectangle, which is 40.