In previous lessons, students added within 100 using place value understanding and properties of operations. They described adding tens to tens and ones to ones without composing a new ten. In previous units, students added within 20 using methods such as counting on, making ten, or using known facts.

In this lesson, students add a one-digit number to a two-digit number in a way that makes sense to them. Students explain their own methods and connect different methods. Students may notice that the value of the sum has more tens than the two-digit addend. They connect this understanding to methods that explicitly show composing a new unit of ten from 10 ones.

Activity 3 is an optional activity that encourages students to consider contexts in which we need to add a two-digit and a one-digit number. When students contextualize and decontextualize addition expressions, they reason abstractly and quantitatively (MP2).

Reflect on who participated in math class today. What assumptions are you making about those who did not participate? How can you leverage each of your students’ ideas to support them in being seen and heard in tomorrow’s math class?