This Problem Set is a part of the Lesson 21, Unit 1, Grade 1. In this lesson, students add doubles (1 + 1, 2 + 2, 3 + 3, etc.) and doubles plus 1 using fingers and 5-group cards. For example, they add 3 + 3 to get 6, and realize that adding 3 + 4 is just adding 3 + 3, and then 1 more to get 7.