Lesson objective: Add the areas of rectangles within a rectilinear figure to solve for the area of the whole rectilinear figure.

This lesson helps to build procedural skill with finding the area of rectilinear figures. The area model is used here because it supports decomposing the shapes. This work develops students' understanding that we can decompose rectilinear figures because area is additive.

Students engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) as they use the area model to decompose rectilinear shapes in order to add areas of rectangles.

**Key vocabulary:**

- area
- area model
- square units

**Special materials needed:**