Understand the formula for calculating the perimeter of a rectangle. This particular shape is defined as a closed figure with four right angles and two pairs of parallel sides. The sides that make up each pair are equal, but the two pairs are not necessarily equal to each other. Thus the formula for the perimeter, the sum total of all sides, of a rectangle would be 2L + 2W = P where "L" is the measurement of the length of one pair of sides, "W" is the measurement of the width of the other pair of sides and "P" is the perimeter.
Read the problem carefully and rewrite the equation using the information provided. If only information about the value of perimeter is given, the problem is unsolvable. It is necessary to also have information about the exact value of one of the sides, or information about the measurement of one of the sides in relation to the other. In the case of the latter, replace the appropriate unknown variable with its relationship to the other unknown variable. For example, if a problem is given in which the perimeter of a rectangle is equal to 120 inches and the width is twice the measurement of the length, the equation would be rewritten: 2(L) + 2(2L) = 120.
Rewrite the equation once more so you're solving for the unknown side. In the case of the example, the equation would be written L = 120/6.
Calculate your answer. L is equal to 20 inches in this example problem.
Solve for the value of the other unknown side, if this information isn't already provided. In the example problem, W = 2(L). Plug the value of L into the equation: W = 2(20). Calculate the answer; W = 40 inches.
Double-check the accuracy of your answers by plugging all values into the perimeter formula. The sum total of the sides should be equal to the value of the perimeter.