Lesson objective: Add two three-digit numbers where it is necessary to regroup 10 ones to make a new ten, or 10 tens to make a new hundred.

This lesson helps to build fluency with whole number three-digit addition with regrouping. Base ten blocks and number lines are used here because they support strategies related to place value and decomposition of numbers. This work develops students' understanding that addition strategies with regrouping used with two-digit numbers can also be used with three-digit numbers.

Students engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) as they apply regrouping strategies from two-digit addition to three-digit addition, using base ten blocks and number lines as tools to support multiple strategies related to place value and properties of operations.

**Key vocabulary:**

- base ten blocks
- compose
- decompose
- digit
- equivalent
- hundreds
- number line
- ones
- place value
- regroup
- tens

**Special materials needed:**

- Number cubes (1 per pair of students)