A restaurant hall is 20 metres long, 15 metres wide and 5 metres high. Its interior has to be covered with wallpaper. What will be the total expenditure if it costs $60 per square metre of wallpaper?
Assuming you want only the walls covered, you have two pairs of walls, with sizes 20x5 and 15x5.
So, add up the areas to get the m^2, and multiply by the cost per m^2.