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:
As students work with modeling and visualizing 3-digit subtraction, their focus should be on understanding the subtraction process and finding strategies based on place value to subtract 3-digit numbers. When done correctly, the traditional approach (algorithm) to subtraction always yields the right answer, but it is not easy for students to understand. Although it is efficient, it focuses on digits and memorized procedures rather than an understanding of the numbers. They will eventually learn the traditional method (algorithm), but emphasis in 2nd grade should be on helping them understand the process.
As students transition to subtracting with greater numbers, they should revisit the strategies that were discussed when working with 2-digit numbers, such as using place value strategies, visualizing with number lines, thinking addition to count up, and regrouping. Understanding place value helps students recognize that hundreds are subtracted from hundreds, tens are subtracted from tens, and ones are subtracted from ones. Breaking numbers apart into expanded form simplifies the subtraction process. Students will also extend their experiences with regrouping from 2-digit numbers. By using trades and renaming numbers using models, students should make sense of the regrouping process. For example, with 453 – 272, they might rename 453 as 3 hundreds, 15 tens, and 3 ones (453 = 300 + 150 + 3).
GCG 1 – Learning Goal: Use place value to subtract 3-digit numbers
- Step 1: Prove that the values are the same when you regroup
- Step 2: Use base-ten blocks and quick pictures to subtract within 1000
- Step 3: Use expanded form to find differences
GCG 2 – Learning Goal: Use number lines and compensation to subtract 3-digit numbers