Lesson objective: Fluently subtract a given number from 13.

This lesson helps to build fluency with mental calculations. Mental strategies are used here because it helps students understand that problems can often be worked out in several different ways. Which method is the best will depend on the numbers involved and the range of methods that students are confident with. This work develops students' understanding that knowing some addition and subtraction facts from memory helps to solve new addition and subtraction problems.

Students engage in Mathematical Practice 1 (Make sense of problems and persevere in solving them) as they draw from their own toolkit for solving problems and utilize strategies that have proven to be efficient for them. In this lesson, it will take the form of children determining what strategy would work best for the given situations. Mathematial Practice 6 (Attend to precision) is also a critical practice within this task. Students are now applying strategies that they have been practicing all year. They are focusing on the precision of communication in speech, as well as in written form. They will be explaining, justifying, and supporting their choice of strategies as they discuss their reasoning with peers.

**Key vocabulary:**

- efficiently
- fluently
- mental calculations

**Special materials needed:**

- deck of playing cards