Lesson objective: Extend understanding of adding and subtracting three-digit numbers to include regrouping.

Students bring prior knowledge of adding and subtracting three-digit numbers from 2.NBT.B.9. This prior knowledge is extended to problems that require regrouping as students regroup 10 tens to make a new hundred, or 10 ones to make a new ten. A conceptual challenge students may encounter is understanding equivalent forms of three-digit numbers.

The concept is developed through work with base ten blocks, which supports the idea that numbers can be regrouped in different ways to add and subtract.

This work helps students deepen their understanding of equivalence because students need to be flexible with equivalent forms of three-digit numbers and represent these numbers in different ways (e.g., 300 is equivalent to 3 hundreds, or 2 hundreds and 10 tens).

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.

**Key vocabulary:**

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