# 10. Comparing strategies of addition and subtraction to look for efficiency (A)

teaches Common Core State Standards CCSS.Math.Content.3.NBT.A.2 http://corestandards.org/Math/Content/3/NBT/A/2
teaches Common Core State Standards CCSS.Math.Practice.MP7 http://corestandards.org/Math/Practice/MP7

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Lesson objective: Compare strategies for adding and subtracting to look for efficient strategies.

This lesson provides an opportunity for students to apply their knowledge and understanding of place value and decomposition to a mathematical situation. Students are asked to compare student strategies to look for efficient strategies of addition and subtraction.

Key Concept students will use:

• Use place value understanding and decomposition to add and subtract using a variety of strategies.

Skills students will use:

• Use concrete models and drawings to understand that it is sometimes necessary to compose and decompose numbers when adding and subtracting. (Grade 2, Unit 11)
• Use counting up (recognizing the inverse relationship of addition and subtraction) to solve subtraction problems.
• Using decomposing to strategically count up or compensate numbers to make adding or subtracting more efficient.

Students engage in Mathematical Practice 7 (Look for and make use of structure) as they decompose numbers using their place values, and they use those values in different ways to most efficiently solve a subtraction problem.

Key vocabulary:

• compensation
• decompose
• difference
• efficient

Special materials needed:

• base ten blocks
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