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2-N-2-4

Page history last edited by Tashe Harris 6 years, 2 months ago

2.N.2.4 Use strategies and algorithms based on knowledge of place value and equality to add and subtract two-digit numbers.

In a Nutshell

Students will continue developing a deep understanding of the base ten number system and place value in adding and subtracting two-digit numbers using a variety of strategies. In third grade, students will use this knowledge to add and subtract multi-digit numbers.

Student Actions	Teacher Actions
Develop strategies for solving addition and subtraction of two digit numbers using multiple representations (e.g., verbal, physical, symbolic, pictures, number line). Develop a deep and flexible conceptual understanding of adding and subtracting two digit numbers through learning a variety of strategies (e.g., counting on, making tens, decomposing numbers, using base-ten blocks). Develop mathematical reasoning by exploring, comparing, and communicating a variety of addition and subtraction strategies(e.g., in small groups and during whole class discussion)	Use and connect among various representations and strategies of adding and subtracting two digit numbers. Elicit and use evidence of student thinking of adding and subtracting to assess progress toward understanding and to support and extend learning. Facilitate meaningful mathematical discourse among students to build shared understanding of adding and subtracting two digit numbers by analyzing and comparing approaches and arguments.
Key Understandings	Misconceptions
There are a variety of strategies to solve two-digit addition and subtraction problems (e.g., counting on/counting back, counting all, drawing pictures, using base ten blocks, rounding, chunking, making tens, adding tens first then adding ones second, etc.).	The Standard Computational Algorithm (SCA, i.e., the method of putting one number on top of the other then adding or subtracting) is the most efficient for adding and subtracting. (Introducing the SCA too early could be detrimental to a student’s math development.) When using the SCA, it's okay to "subtract up" when the digit in the minuend is smaller than the digit in the subtrahend. For example, 56 - 37, the student may subtract 6 from 7 and get 1. Then subtract 3 from 5 and get 2, for an answer of 21.