Lesson objective: Understand and solve three-digit subtraction problems, with regrouping, as missing addend problems.

Students bring prior knowledge of adding and subtracting three-digit numbers from 2.NBT.B.9. This prior knowledge is extended to solving missing addend problems using a variety of addition or subtraction strategies. A conceptual challenge students may encounter is understanding the relationship between addition and subtraction.

The concept is developed through work with tape diagrams, which helps students see parts of a whole to support them in the understanding that subtraction problems help us find a missing part of the whole.

This work helps students deepen their understanding of operations because the relationship between addition and subraction can be used as a strategy to solve problems.

Students engage in Mathematical Practice 1 (Make sense of problems and persevere in solving them) as they find a missing addend by exploring both addition and subtraction strategies. Tape models support students' understanding of subtraction problems as missing addend problems.

**Key vocabulary:**

- addend
- base ten blocks
- decompose
- digit
- equivalent
- hundreds
- number line
- ones
- place value
- regroup
- tape diagram
- tens