Question 126504
The area of your original piece of paper was 100 X 80 = 8000 cm^2.  You removed 4 pieces each of which had an area of 100 cm^2 (10 X 10).  So you removed 400 cm^2.  8000 - 400 = 7600 cm^2.  That gives you the surface area of either the outside of the box or the inside of the box.  If you are considering the total surface area to be both the inside and the outside, just multiply 7600 by 2.