Kelly wants to create a rectangular playpen for his cats in his backyard, and estimates that the cats will need 500 square feet. He also wants the length of the playpen to be 20 feet longer than the width, so they have room to run. What dimensions should the playpen be?

You can solve this problem algebraically by representing each of the unknown values (the dimensions of the playpen or its length and width) with certain variables. Then, you represent the length in the terms of the width, as it is 20 feet longer than the length. What you may do next is substitute all of the parts of the equation that you've composed into the area formula for the rectangle, and then you can solve for the width and then for the length of the playpen, hence finding its dimensions.