The purpose of this activity is for students to explore the relationship between addition and subtraction through a Compare, Difference Unknown story problem. They analyze two equations, one addition and one subtraction, that match the same problem and discuss the relationship between the two equations and the story problem. Students should notice that both equations can be used to describe how to solve the problem. This helps students relate addition and subtraction and see that often either operation can be used to solve a problem (MP7). This also helps students understand subtraction as a unknown addend problem, which will be helpful as students make sense of computation strategies within 20 in upcoming lessons.

• “What are we trying to find out in this story problem?” (How many fewer scissors there are than glue sticks. The difference between the number of scissors and glue sticks.)
• Invite previously identified groups to share.
• “Why can we use either equation to represent finding the difference or how many fewer?” (When you find the difference, it’s like thinking how much you can add to the smaller amount to get the bigger amount. You can find the difference by taking away the smaller amount from the bigger amount. Subtraction is a way to find an unknown addend.)

In the previous lesson, students matched Add To, Take From, and Put Together/Take Apart problems to equations and made sense of the way an addition and a subtraction equation can be used to represent a Put Together/Take Apart, Addend Unknown problem. This matching task gives students an additional opportunity to analyze the Compare, Difference Unknown problem structures and equations closely and make connections (MP2). As students work, encourage them to refine their descriptions of how different equations represent the actions or the relationships between quantities (MP6). In particular, look for the way students make connections to the bigger quantity, smaller quantity, and the difference.

The set of cards also includes an Add To, Change Unknown problem. There is only one equation card that matches this problem in the set of cards provided. However, in the Lesson Synthesis, students will discuss whether or not a subtraction equation could be used to represent the problem. This discussion is a foundation for students' algebraic thinking. A subtraction equation does not represent the actions in the problem, but can be used to solve it based on the relationship between addition and subtraction.

• Invite previously selected students to share for Cards F and G.
• Highlight the ways students identify the bigger quantity, smaller quantity, and difference.
• “When we compare, the equations do not represent an action. They represent a way to show how a bigger amount and smaller amount are related. We can use addition or subtraction to show that relationship.”