# 7. Finding unknown values in addition and subtraction situations (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 that we can find unknown values using what we know about the relationship between addition and subtraction. Extend that understanding to problem solving situations.

Students bring prior knowledge of using addition and subtraction for situational problem solving tasks from 1.OA.A.1. This prior knowledge is extended to a greater number set as students work on problems with unknowns in a variety of positions. A conceptual challenge students may encounter is understanding what the unknown is.

The concept is developed through work with a number bond representation, which will help students visualize the unknown in the situations.

This work helps students deepen their understanding of operations because students will extend their knowledge  of addition or subtraction to solve problems with unknown addends.

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. Completing and creating number bonds to model the math situations will support this work.

Key vocabulary:

• number bond
• part
• whole

Special materials needed:

• place value tools (rods, units)
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