Lesson objective: We can use the relationship between addition and subtraction to add and subtract within 1000.

Students bring prior knowledge of developing strategies for solving addition and subtraction from 3.NBT.A.2. This prior knowledge is extended to using these strategies to solve word problems as students use their understanding of addition and subtraction as inverse operations. A conceptual challenge students may encounter is deciding when it is most efficient to use subtraction to solve an addition problem or addition to solve a subtraction problem.

The concept is developed through work with an open number line, which models how to solve an addition problem or addition to solve a subtraction problem.

This work helps students deepen their understanding of operations because they use place value strategies, properties of operations, and the relationship between addition and subtraction to solve addition and subtraction problems.

Students engage in Mathematical Practice 2 (Reason abstractly and quantitatively) as they create logical representations of the problem at hand and use what they know about the relationship between addition and subtraction to choose which operation would be the most efficient.

**Key vocabulary:**

- addition
- inverse operations
- open number line
- subtraction