Primary Standards:

MAFS.2.NBT.2.7 Add and subtract within 1000, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method.  Understand that in adding or subtracting three digit numbers, one adds or subtracts hundreds and hundreds, tens and tens, ones and ones; and sometimes it is necessary to compose or decompose tens or hundreds.

MAFS.2.NBT.2.8 Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900.

MAFS.2.NBT.2.9 Explain why addition and subtraction strategies work, using place value and the properties of operations.

Connecting Standards:

MAFS.2.OA.2.2 Fluently add and subtract within 20 using mental strategies.  By end of Grade 2, know from memory all sums of two one-digit numbers.

Content Knowledge:

In this Unit, students extend their knowledge of addition with 2-digit numbers and the regrouping process (when it is necessary or not to create a new ten) to work with 3-digit numbers.  Continuing with the structure of adding ones with ones, and tens with tens, they now add hundreds with hundreds, and realize that sometimes it is necessary to make a new hundred.  Students should continue to make use of concrete base-ten models to further their understanding of the addition process.

As students transition to adding larger numbers, they revisit concepts and strategies used with 2-digit numbers, such as modeling with base-ten blocks, partial sums, decomposing to count up, etc.  By exploring and discussing varied strategies, students continue to deepen their understanding of the addition process.

Students should spend time exploring strategies based on their understanding of place value, rather than memorizing a traditional algorithm for adding 3-digit numbers, because the traditional algorithm focuses on digits and memorized procedures rather than an understanding of the numbers and the process.

Informal use of Number Talks can be a fantastic connection to allowing students to use mental strategies based on place value and properties of operations.  Number Talks concepts do not necessarily need to parallel the work done in the current Unit(s), and instead may focus attention on prior concepts which could help to develop fluency with computations, such as using compensation and/or decomposing with 2-digit numbers, or basic fact strategies.


GCG 1 – Learning Goal:  Use place value to add 3-digit numbers

  • Step 1: Add 3-digit numbers using base-ten blocks and quick pictures
  • Step 2: Add 10 or 100 to a 3-digit number
  • Step 3: Break apart 3-digit numbers by place value (expanded form) to add
  • Step 4: Add 3-digit numbers using partial sums

GCG 2 – Learning Goal:  Use number lines and compensation to add 3-digit numbers

  • Step 1: Use place value to add 3-digit numbers on an open number line
  • Step 2: Use compensation to add 3-digit numbers
  • Step 3: Compare and explain strategies used to add 3-digit numbers