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.
