The triangular surface of the shaded part is half of the parallelogram, and the areas of the two blank triangles are half of the parallelogram. It can be assumed that the base of the parallelogram is a, the height is h, and the area of the triangle is ah*( 1/2). It is known that the triangular surface is half of a parallelogram. Triangular surface of shaded part = area of two blank triangles, 15+25=40 (square centimeter). Questions like this should be in the fourth grade of primary school.