An esky has a height of 50cm, a width of 25cm and a length of 75cm. The walls of the esky are 4cm thick on each side (including the lid). If it is filled with cold water, how many litres will it hold?
The answer is apparently 236cm², and apparently you have to subtract 8 from the height, length and width. I don't really understand why.
To show why we drop 8 cm on each dimension, let's sketch out one side of the esky*:
(* Not entirely to scale...)
The surface area of this side is 75x50 $cm^2$. The blue area is (75 - 4 - 4) x (50 - 4 - 4) = 67 x 42 $cm^2$. Don't mind the orange areas, you're not subtracting them twice. You can see that a little more clearly when we cheat a bit and move the blue area:
This is done on a 2D-plane to show the principle, but it holds for 3D volumes as well.