The perimeter of a rectangle can be found using the formula: P = 2W + 2L, where P = perimeter, W = width and L = length. Since we know the value of two of the three variables, we can plug them into the equation and solve for the missing value ('L'): 400 = 2(80) + 2L or 400 = 160 + 2L. Using inverse (opposite) operations to isolate the variable, we get: 400 - 160 = 160 - 160 + 2L or 240 = 2L. We then divide both sides of the equation by 2 to get rid of the coefficient: 240/2 = 2L/2 or 120 = L. So, the length of the rectangle is 120 feet.
