the answer found from the web:
the smallest angle of the red triangle = inverse tan( 3/8 ) = 20.56
degrees
the smallest angle of the green triangle = inverse tan( 2/5 ) =
21.8 degrees
So the two triangles used to form the hypotenuse of the final
"triangle" (all four pieces total) don't actually make a straight
line, which causes the final "triangle" to be a quadrilateral.
Depending on which of the smaller triangles is used on top and on
bottom changes the total area of the quadrilateral, and when the
red one is on top there is enough area added to equal one extra
square 
