This Fluency Activity is a part of the Lesson 10, Unit 4, Grade 4. This fluency exercise prepares students for unknown angle problems in Lessons 10 and 11. Project a number bond with a whole of 90. Fill in 45 for one of the parts. On your personal white boards, write the number bond, filling in the unknown part. Continue to break apart 90 with the following possible sequence: 35, 25, 65, and 15.