The purpose of this lesson is for students to make sense of and solve a new type of problem—Add To, Change Unknown. Students solve the problems in a way that makes sense to them and identify the answer to a story problem in their work.

In previous lessons, students represented Add To and Take From, Result Unknown story problems in different ways. They connected their representations and strategies to the actions in the problems. In this lesson, students are introduced to Add To story problems where the change is unknown. Students represent and solve the problems in a way that makes sense to them and discuss different ways to show their answer. They compare Add To, Change Unknown and Add To, Result Unknown problems and discuss how the differences affect how they represent and solve the problems.

In the Lesson Synthesis, students are invited to compare Add To, Result Unknown and Add To, Change Unknown by describing the differences in what is unknown. Students are introduced to the terms “addend,” “unknown,” and “unknown addend,” but are not expected to produce these words. They also make sense of using to represent an unknown in an equation. Students will continue to make sense of equations throughout the unit, but they are not expected to produce equations on their own until the end of the unit.