Lesson objective: Understand that there are different ways to decompose numbers to aid in adding or subtracting.

Students have prior knowledge of using concrete models and drawings to demonstrate that it is sometimes necessary to compose and decompose numbers when adding and subtracting from 2.NBT.B.7. This prior knowledge is extended to working with addition and subtraction of numbers to 1000 as students use place value understanding to add and subtract using a variety of strategies. A conceptual challenge students may encounter is that there is only one way to add or subtract.

The concept is developed through work with base ten blocks, which allow students to justify correct sums and differences.

This work helps students deepen their understanding of operations because they will use decomposition of numbers in different ways to make subtracting simpler.

Students engage in Mathematical Practice 6 (Attend to precision) as they see that when adding or subtracting it is important to work correctly with the values set in the original problem and when decomposing a number into expanded notation using place value correctly, one must still attend to the precision of the values as determined by the operation.

**Key vocabulary:**

- decompose
- difference
- efficient

**Special materials needed:**

- base ten blocks