This Instructional Activity is a part of the Lesson 22, Unit 3, Grade 7. Consider providing interlocking cubes to small groups of students so they may construct a model of the diagram shown. Remind students to count faces systematically. For example, first consider only the 9 bottom cubes. This structure has a surface area of 30 (9 at the top, 9 at the bottom, 3 and on each of the four sides).