be “well-defined” the collection description would have to settle all such questions. ... All these questions indicate the statement is ambiguous, i.e., it is not clear which students are members of this collection, hence, the collection is not well-defined.
This is clear because the entire shape is a square; meaning all of it´s sides are equal. For instance if you cut the square in half you would get congruent rectangles meaning they are the same size. The same would happen is you cut the square diagonally, which is what happened. That is why the triangles are congruent. (Hope this helps :3)