TWO TRIANGLES IN A RECTANGLE

At first I thought the answer should be C.
but then it is A , but I am sure I would have got it wrong if I hadn't seen the OA

the area of the shaded region will be 1/2 * the area of the rectangle. Always.
just suppose any value x and l-x for the bases of the triangles and you will always get 1/2 * the area of rectangle

Yes OA A

We know the area of Rectangle =54
Just concentrate on the unshaded part of the Rectangle ,the unshaded part is nothing but a triangle whose area = (1/2)* base *height

The (base* height) is nothing but the area of rectangle.
Thus the area of unshaded portion = Area of rectangle -
(1/2) Area of rectangle

Nice one!