This Problem Set is a part of the Lesson 30, Unit 4, Grade 6. Students calculate missing angle measures by writing and solving equations. 4.MD.C.5 Recognize angles as geometric shapes that are formed wherever two rays share a common endpoint, and understand concepts of angle measurement: a. An angle is measured with reference to a circle with its center at the common endpoint of the rays, by considering the fraction of the circular arc between the points where the two rays intersect the circle.