This Problem Set is a part of the Lesson 20, Unit 7, Grade 3. In this lesson, students find the possible areas of rectangles with a given perimeter. (For example, they are asked to build rectangles with a perimeter of 12 unit squares and divide 12 by 2 to find that the sum of the length and width is 6. From there, they determine that they can make three rectangles whose lengths and widths add up to 6, which results in rectangles made with 5, 8, and 9 unit squares.)