The slope of the line drawn from the top left vertex of the square to the bottom right vertex of the triangle = -1/2
The equation of this line is y = (-1/2)x + 10 (1)
The point at the top vertex of the triangle is (15, 10)
The slope of the segmnent joining (10,0) and (15,10) = 10/5 = 2
So....the equation of the line joining these two points is y = 2(x - 10) ⇒ y = 2x - 20 (2)
The x coordinate of the intersection of these lines can be found by setting (1) = (2)
(-1/2)x + 10 = 2x - 20
30 = [5/2] x
x = 12 and y = 2(12) - 20 = 4
So....the height of the triangular shaded area = 4 and its base = 10
So...its area = (1/2)(4) (10) = 20 units^2