This Problem Set is a part of the Lesson 9, Unit 4, Grade 3. In this lesson, students cut apart rectangular grids and rearrange the parts to create new rectangles with the same area. They conclude that area is additive: a 10-cm by 10-cm rectangle, for example, has the same area as two 5-cm by 10-cm rectangles, or five 2-cm by 10-cm rectangles.