Area of rectangle is ah, where h is the common height of the rectangle and trapezoid, and a is the common length of the larger base of the trapezoid and the rectangle. The smaller base of the trapezoid is a/2. The area of the trapezoid is ah/2 (the area of the rectangle made up of the trapezoid's shorter base and its height, h)+the areas of the triangular "wings". If we call the bases of the triangles x and y, we know that x+y=a/2 because the longer base of the trapezoid is a/2. The height of both triangular wings is h and their total area is xh/2+yh/2=h(x+y)/2=ah/4. So the total area of the trapezoid=ah/2+ah/4=3ah/4. If the rectangle keeps the same height h but changes its base length, b, to make the areas the same, then hb=3ah/4, so b=3a/4 compared to a, which it was before. So the reduction is a/4 or 25%.