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.2.9 Explain why addition and subtraction strategies work, using place value and the properties of operations.
Connecting Standards:
MAFS.2.NBT.2.8 Mentally add 10 or 100 to a given number 0-90 100-900, and mentally subtract 10 or 100 from a given number 10-100 100-900.
Content Knowledge:
Students in 1st grade have worked with some subtraction of tens and ones. In 1st grade students added 2-digit numbers with 1-digit numbers, and two 2-digit numbers. Students have mentally subtracted ten from any 2-digit number, and also subtracted multiples of tens from multiples of tens.
In 2nd grade, students will continue and extend their work with addition and subtraction of 2-digit numbers. They will continue to use place value understanding to make sense of these operations. Students will build upon strategies used with addition in 1st grade to now visualize the subtraction process with manipulatives (base-ten blocks, snap cubes, counters on ten frames), hundred charts, and number lines.
As students work with modeling and visualizing 2-digit subtraction, their focus should be on understanding the subtraction process and finding strategies to subtract 2-digit numbers. When done correctly, the traditional approach (algorithm) to 2-digit 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) with fluency in 4th grade, but emphasis in 2nd grade should be on helping them understand the process.
GCG 1 – Learning Goal: As a mathematician, I can Use base-ten models and quick pictures to subtract two-digit numbers MAFS.2.NBT.2.5, MAFS.2.NBT.2.9
- Step 1: Use base-ten blocks to subtract two-digit numbers
- Step 2: Use a quick picture drawing to subtract two-digit numbers
- Step 3: Use a break-apart strategy to subtract two-digit numbers
GCG 2 – Learning Goal: As a mathematician, I can Use open number lines to subtract two-digit numbers MAFS.2.NBT.2.5, MAFS.2.NBT.2.9