# 10. Solve math stories by identifying unknowns (A)

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: This lesson provides an opportunity for students to apply their knowledge and understanding that we can find unknown values in addition or subtraction situations using the relationship between addition and subtraction.

Students will solve math problems by finding the value of unknowns in a real-life situation. Students are asked to help a grocery store owner figure out a variety of problems with her stock. Students will use structures of their choice, such as number bonds or tape diagrams, to figure out the unknown.

Key Concept students will use:

• We can find unknown values in addition or subtraction situations using the relationship between addition and subtraction.

Skills students will use:

• finding unknowns using tape diagrams
• finding unknowns using number bonds
• computation skills (which may include counting on or back using open number lines or hundreds charts; decomposition; or models such as drawing or using place value tools)

Students engage in Mathematical Practice 1 as they make sense of problems and persevere in solving them, which is central to the work in this task. Students will need to determine the meaning of the problem and consider entry points that will lead to a solution. Students may rely on pictures or models to help conceptualize the problem. They understand that there may be multiple ways to find the solution. As they work through the task, students should be asking themselves, “Does this make sense?”  Students will determine unknown values and work to apply strategies to find solutions using models or representions.

Key vocabulary:

• equation
• number bond
• tape diagram

Special materials needed:

• hundreds charts
• place value tools
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