In this lesson, students apply the reasoning they used in previous lessons of addends that make 10, to find the numbers that make equations true that include the number 20. In the first activity, students learn Stage 3 of the What’s Behind My Back? center in which they work with 20 connecting cubes, organized into two towers of 10 connecting cubes. In the second activity, students find the number that makes different equations with addition and subtraction expressions true. Throughout the lesson, students have opportunities to use what they know about the structure of whole numbers and the relationship of addition and subtraction to find the unknown numbers and explain their methods (MP3, MP7).

Tell students that, at the end of the lesson, they will be asked to identify specific actions from their “Doing Math” list they personally experienced.