All side lengths of a square are equal. The three squares create a right triangle, and the hypotenuse of this triangle represents the side length of the square.
All right triangles must follow the Pythagorean Theorem where is the hypotenuse of the triangle.
Therefore, let be the side length of this largest square. Since the area of a square with side length is given by , will represent the area of the square:
She has 15 left because she had 28 then she at 3 which is now 25 then she gave 5 away which is 20 then she gave 5 more away which is 15. So she ends up with 15