An open-top rectangular water tank is to be made from sheet metal. It is 3 m long, 2 m wide and 1.5 m deep, and it has no lid. What area of sheet metal is needed, in square metres?

An open tank needs the base plus four walls: base = $3\times 2=6$ sq m and walls = $2(l+b)h=2(3+2)\times 1.5=15$ sq m, so 6 + 15 = 21 sq m. The trap 27 is the closed cuboid $2(lb+bh+hl)$, which pays for a lid the tank does not have; 15 is the four walls with the base left out; 13.5 drops the 2 from the wall formula, using $(3+2)\times 1.5=7.5$ sq m for all four walls.