if rectangle abcd, then the surface of rectangle is ==> ab*ac=cd*bd=ab*bd=cd*ac.
A rectangle is divided into 2 equivalent triangles (abc and adc), then the surface of triangles are (ab*bc)/2 and (ad*cd)/2.
so the surface of every equivalent triangles is half of main rectangle, so 6.7/2=(6 7/10)/2=(67/10)/2=67/20=335/100=3.35