Mental Calculation.

The goal of this exerise is to delineate MULTIPLE ways to mentally calculate the area of the yellow region in the figure.

A square is construct with side length equal to the length of the hypotenuse of a right triangle situation in one of its corners. The right triangle has legs of length 10 units. What is the area of the region of the square outside of the triangle?

Even if you grab a pencil and paper, it is still within the aim of this exercise to describe the process you use to determine the area.