In previous lessons, students added and subtracted within 20 with teen numbers that did not require composing or decomposing a ten. They used counting on, take away, the structure of teen numbers, and the relationship between subtraction and addition.

The purpose of this lesson is for students to solve a new type of story problem—Take From, Change Unknown. Students also solve a familiar problem type—Take From, Result Unknown. They use methods that make sense to them and then make connections between methods.

Think about who volunteered to share their thinking with the class today. Are the same students always volunteering, while some students never offer to share? What can you do to help the class understand the value of hearing the ideas of every mathematician?