# 4. Subtract within 100 using place value understanding (C)

teaches Common Core State Standards CCSS.Math.Content.2.NBT.B.5 http://corestandards.org/Math/Content/2/NBT/B/5
teaches Common Core State Standards CCSS.Math.Content.2.OA.A.1 http://corestandards.org/Math/Content/2/OA/A/1
teaches Common Core State Standards CCSS.Math.Practice.MP1 http://corestandards.org/Math/Practice/MP1
teaches Common Core State Standards CCSS.Math.Practice.MP7 http://corestandards.org/Math/Practice/MP7

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Lesson objective:  Understand how knowing about tens and ones can help solve subtraction problems.

Students bring prior knowledge of place value from 1.NBT.1. This prior knowledge is extended to subtraction as students model subtraction of two-digit numbers using place value tools. A conceptual challenge students may encounter is decomposing tens.

The concept is developed through work with place value rods and units or drawings, which helps students understand and reinforce that ten ones is equal to one ten, and that is why we can decompose a ten when necessary.

This work helps students deepen their understanding of numbers because students represent relationships among numbers by composing and decomposing 10s and 1s to add and subtract.

Students engage in Mathematical Practice 7 as they look for and make use of structure, which is central to the work in this task. Students should identify the structure of place value in order to add or subtract within 100. Using models and tools helps students to solidify thier understanding.

Key vocabulary:

• decompose
• difference
• place value

Special materials needed:

• place value tools (rods and units)
• unifix cubes (alternatives to rods and units)
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