The diagonal lines of the isosceles trapezoid are equal. Therefore, from "E, F, G, H are the midpoint of each side of the isosceles trapezoid ABCD", we can know that the quadrilateral EFGH is a diamond, the area of the diamond is half of the product of its diagonal, and its diagonal is just the middle line of the height and waist of the isosceles trapezoid; According to the formula for calculating trapezoidal area, the area of trapezoidal ABCD is 20*2=40.