In this unit, students build on previously developed strategies for modeling addition and subtraction. Students will begin the unit by adding and subtraction within 20. They will gradually work their way up to adding and subtracting larger values within 100, by extending their thinking to include addition and subtraction to 100 using mental strategies. Understanding of place value supports students as they add and subtract twodigit numbers using a variety of strategies including decomposition. Students extend their knowledge of addition or subtraction to solve problems with unknown addends. Students learn to compare known quantities by using their knowledge of addition and subtraction operations to find an unknown value.
Key Concepts:
 We can add numbers within 100 using our knowledge of place value.
 We can subtract numbers within 100 using our knowledge of place value
 We can find unknown values in addition and subtraction situations using the relationship between addition and subtraction
 We can compare known quantities by finding the unknown value.
Prior Knowledge Needed:
 Add and subtract using strategies such as counting on, making ten, decomposing a number leading to a ten, using the relationship between addition and subtraction, compensating or creating equivalent but easier or known sums; (Grade 1, Unit 1, 1.OA.6)
 Understand that the two digits of a twodigit number represent amounts of tens and ones; (Grade 1, Unit 1, 1.NBT.2)
 Applying properties of operations as strategies to add and subtract; (Grade 1, Unit 6, 1.OA.3)
 Adding multiples of ten; (Grade 1, Unit 11, 1.NBT.4, 1.NBT.5)
 Count forward or backward by ten from a given number.
 Use place value to add and subtract (Grade 1, Unit 14, 1.NBT.C.4 ;1.NBT.C.5; 1.NBT.C.6)
 Apply properties of operations to solve problems (Grade 1, Unit 15, 1.OA.2, 1.OA.3)
Units on the Horizon:
 Relating skip counting to mental addition and subtraction (Grade 2, Unit 8)
 Demonstrating fluency in addition and subtraction (Grade 2, Unit 15) 2.OA.1, 2.OA.2, 2.NBT.5
 Developing strategies for addition and subtraction (Grade 3, Unit 3) 3.NBT.1, 3.NBT.2
 Solving addition and subtraction problems involving measurement (Grade 3, Unit 6) 3.MD.2
Lessons

Lesson objective: Understand how to use addition and subtraction in compare word problems to find the unknown. Students bring prior knowledge of comparing numbers from 1.OA.A.1 This prior knowledge is extended to explaining what comparison situations me...

Lesson objective: Practice writing comparing values using either addition or subtraction. This lesson helps to build fluency with comparison statements. Part/part/whole models and cubes are used here because they allow students to compare values using a...

Lesson objective: Solve real world addition and subtraction comparison problems. This lesson provides an opportunity for students to apply their knowledge and understanding of the meaning of comparison situations. Students are asked to compare the numbe...

Lesson objective: Practice explaining the comparison between known quantities and an unknown value. This lesson helps to build fluency with comparison statements. Number lines, cubes and part/part/whole models are used so students can compare values usi...

Apply understanding of addition and subtraction situations to create compare story problems. This lesson provides an opportunity for students to apply their knowledge and understanding of addition and subtraction comparison situations to a reallife sit...

Lesson objective: Understand that we can add twodigit numbers using our understanding of place value. Students bring prior knowledge of counting by tens for computation from 1.OA.6. This prior knowledge is extended to counting on by both tens and ones ...

Lesson objective: Fluently count on by tens and ones to find sums using base ten blocks and hundred charts. This lesson helps to build procedural skill whole number addition. Base ten blocks and hundreds charts are used here because they support underst...

Lesson objective: Apply understanding of tens and ones to solving addition equations when the addends are decomposed into tens and ones. This lesson provides an opportunity for students to apply their knowledge and understanding of place value to a math...

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 twodigit numb...

Lesson objective: Fluently use an open number line or hundreds chart to find differences by counting back or counting on. This lesson helps to build procedural skills with whole number subtraction. Base ten blocks and hundreds charts are used here becau...

Lesson objective: Apply understanding of place value to determine what strategy can be used most efficiently to solve subtraction problems. This lesson provides an opportunity for students to apply their knowledge and understanding of place value to a m...

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 subtra...

Lesson objective: Practice creating equations by understanding part/part/whole diagrams. This lesson helps to build procedural skill with whole number addition and subtraction. Part/part/whole diagrams are used here because they highlight the relationsh...

Lesson objective: Practice using part/part/whole diagrams to model math situations. This lesson helps to build procedural skill with whole number addition and subtraction. Part/part/whole diagrams are used here because they highlight the relationship be...

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 so...