The four walls of a rectangular room have a combined area of $120{\text{m}}^2$, and the room is 4 m high. Find the perimeter of its floor, in m.

The four walls unroll into one long rectangle as tall as the room and as long as the floor's perimeter, so area of walls = perimeter × height. Perimeter = 120/4 = 30 m. (15 m is l + b, only half the way round; taking the wall area as (l + b)h gives l + b = 30 and a perimeter of 60 m; 480 multiplies by the height instead of dividing.)