Primary Standards:
MAFS.2.NBT.2.5 Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction.
MAFS.2.NBT.2.6 Add up to four two-digit numbers using strategies based on place value and 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.
MAFS.2.OA.1.a Determine the unknown whole number in an equation relating four or more whole numbers. For example, determine the unknown number that makes the equation true in 38 + 10 + 10 = ___ + 18, or ? – 6 = 13 – 4, or 15 – 9 = 6 + □
Content Knowledge:
Students in 1st grade have worked with addition of tens and ones. In 1st grade students added 2-digit numbers with 1-digit numbers, and two 2-digit numbers. They developed the concept that tens and tens can be combined, and ones and ones can be combined, and that sometimes a new ten can be created when more than ten ones are combined (an informal understanding of regrouping). Students have also mentally added ten to any 2-digit number.
Students should spend time exploring strategies based on their understanding of place value, rather than memorizing a traditional algorithm for adding 2-digit numbers, because the traditional algorithm focuses on digits and memorized procedures rather than an understanding of the numbers and the process.
There are multiple strategies for adding 2-digit numbers that are grounded in an understanding of place value and properties of operations. Exploring different ways to add 2-digit numbers provides our students with opportunities to examine methods that work, discuss how they are alike and different, talk about when each might be most effective, and ultimately understand the process more deeply.
Using these strategies students apply and build upon their understanding of numbers, place value, and properties to find reasonable ways to determine the sum. Students use manipulatives, drawings, and number lines to visualize the addition process as they try the strategies.
GCG 1 – Learning Goal: As a mathematician, I can Use base-ten models and quick pictures to add two-digit numbers MAFS.2.NBT.2.5
- Step 1: Use base-ten blocks to add two-digit numbers
- Step 2: Understand the value is the same when base-ten blocks are regrouped to find the sum
- Step 3: Use a quick picture drawing to add two-digit numbers
GCG 2 – Learning Goal: As a mathematician, I can Use other place-value strategies to add two-digit numbers MAFS.2.NBT.2.5
- Step 1: Break apart addends into tens and ones, find partial sums, and combine to find the sum
- Step 2: Use open number lines and benchmarks of ten to add within 100
- Step 3: Use compensation by changing the addends to simplify the addition problem
GCG 3 – Learning Goal: As a mathematician, I can Solve equations with multiple two-digit addends MAFS.2.NBT.2.5, MAFS.2.NBT.2.6