In previous lessons, students recognized the structure of teen numbers and used the structure to find the sum of three numbers. In this lesson, students use the structure of the 10-frame to find the sum of two addends when one addend is 9. Students come to see that when they find the sum of two addends, they can decompose one addend to make 10 with the other addend (associative property). For example, . When students identify and use equivalent expressions, they look for and make use of structure (MP7), and here they repeatedly make 10 to find the value of expressions (MP8).

Student methods are recorded with equations. It may also be helpful to represent how one addend is decomposed in order to make a ten.