A rectangular pit 6 m long and 4 m wide is lined with cement on its floor and on its four vertical walls; the top is left open. The lined area comes to $104{\text{m}}^2$. Find the depth of the pit in m.

Floor = $6\times 4=24{\text{m}}^2$; the four walls = $2(6+4)h=20h$. So $24+20h=104$, giving $20h=80$ and $h=4$ m. (Forgetting the floor gives 104/20 = 5.2 m; lining a lid as well adds another $24{\text{m}}^2$ and gives 2.8 m, but the pit is open at the top; using $(l+b)h=10h$ for the walls, which counts only one wall of each pair, gives 8 m.)