In this unit, students use their place value understanding and number sense to round numbers and to estimate sums and differences. Students will also use place value strategies, properties of operations, and the inverse relationship of addition and subtraction to find sums and differences within 1,000. Students will understand that perimeter of a polygon is the sum of the side lengths, and will find the perimeter given the side lengths. They will solve real world and mathematical problems involving perimeters of polygons. Students will use estimation to judge the reasonableness of answers. Students may struggle with the terms “rounding up” and “rounding down” without a visual model such as a number line or hundred chart.
Key Concepts:
 Rounding is based on place value, and may be used to gauge the reasonableness of calculations or may be used in place of a specific calculation.
 We can use place value understanding to add and subtract using a variety of strategies.
 Perimeter is the sum of lengths of the sides of a polygon.
Prior Knowledge Needed:
 Represent whole numbers on a number line diagram with equally spaced points and recognize the distance and proximity from other numbers; (Grade 2, Unit 3; 2.MD.B.6).
 Use place value understanding and properties of operations to add & subtract; (Grade 2, Unit 11; 2.NBT.B.6).
 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; 2.NBT.B.7).
Units on the Horizon:
 Solving addition and subtraction problems involving measurements (Grade 3, Unit 5: 3.MD.A.1, 3.MD.A.2).
 Investigating patterns in number and operations (Grade 3, Unit 8: 3.NBT.A.1).
 Applying place value concepts in whole number addition and subtraction (Grade 4, Unit 4: 4.NBT.A.3).
Lessons

Lesson objective: Round to the nearest hundred using a number line. Students bring prior knowledge of place value from 2.MD.B.6. This prior knowledge is extended to rounding as students use number lines to determine what value to round to. A conceptual ...

Lesson objective: Round to the nearest hundred using a number line. This lesson helps to build procedural skill with rounding. A number line is used here because it supports understanding of the relational location of values as compared to benchmark num...

Lesson objective: Determine which possible numbers will round to a given value. This lesson helps to build procedural skill with rounding. A number line is used here because it supports the concept that numbers that are close to benchmark numbers can ro...

Lesson objective: Extend understanding of estimation to determine reasonability of sums and differences in realworld contexts. Students have prior knowledge of estimation from 3.NBT.A.1. This prior knowledge is extended to addition and subtraction as s...

Lesson objective: Apply understanding of estimation to determining reasonability of sums and differences in realworld contexts. This lesson provides an opportunity for students to apply their knowledge and understanding of estimation to a reallife sit...

Lesson objective: Understand that there are different ways to decompose numbers to aid in adding or subtracting. Students have prior knowledge of using concrete models and drawings to demonstrate that it is sometimes necessary to compose and decompose n...

Lesson objective: Add numbers within 1000 by decomposing values to make regrouping easier. This work develops students' understanding that the order of addends when adding will not affect the sum. Students engage in Mathematical Practice 7 (Look for and...

Lesson objective: Add numbers within 1000 by decomposing and using a number line. This lesson helps to build procedural skills with whole number addition. A number line is used here because it supports tracking of multiple addends and looking for effici...

Lesson objective: Subtract within 1000 using decomposing and counting up to solve a subtraction problem. This lesson helps to build procedural skill with whole number subtraction. A number line is used here because it supports the understanding that cou...

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. S...

Lesson objective: Understand that perimeter is a measure of length calculated by adding the lengths of all of the sides of a polygon. Students bring prior knowledge of addition strategies from 2.NBT.5. This prior knowledge is extended to calculating per...

Lesson objective: Find the perimeter of a polygon marked with unit length markers, but without numbers. This lesson helps to build procedural skill with determining perimeter. Polygon models are used here because they provide a visual that supports the ...

Lesson objective: Find the perimeter of a polygon with given side lengths. This lesson helps to build procedural skill with determining perimeter. Polygon models are used here because they provide a visual that supports the skill of finding perimeter. M...

Lesson objective: Understand that perimeter is not additive, i.e., the perimeter of a composite figure formed by smaller polygons is not the sum of their perimeters. This lesson helps to build procedural skill with calculating perimeter of composite sha...

Lesson objective: Generate side lengths for possible rectangles that have a given perimeter. This lesson provides an opportunity for students to apply their knowledge and understanding of perimeter to a reallife situation. Students are asked to create ...