This Instructional Activity is a part of the Lesson 13, Unit 2, Grade 8. Provide students 15 minutes of work time. Once the 15 minutes have passed, review the solutions with students. As before, our goal is to show that the sum of the measures of the interior angles of the triangle are equal to the measure of the straight angle. Use what you learned from Exploratory Challenge 1 to show that the measures of the interior angles of a triangle have a sum of 180 degree.