Before You Begin this Unit…  
Unit at a Glance 
This unit builds on the ideas from Unit 1, with students becoming familiar with mathematical tools, and developing strategies for accurately counting a set of objects. They are also continuing to become fluent with addition combinations. A large focus is also on making sense of addition and subtraction story problems. In this unit, start unknown and change unknown situations are introduced. It is important to note that the numbers used in the story problems of Investigation 2 in this unit are kept small so that students can focus on the situation. Later, students will work on similar problems with larger numbers. Even though many students do not need cubes to solve a story problem, it is important that everyone understands how to model the situation with cubes. In this unit, most students are in the process of making a critical shift from thinking and working primarily in 1s, to thinking and working in groups of 10. By organizing objects into groups of tens and ones, students experience that the number in the tens place represents the groups of 10 and the number in the ones place represents the number of ones. 
Suggested Dates: *Investigation 1: 6 lessons *Common Core Unit 2 Session 1.1A*Investigation 2: 8 lessons *Investigation 3: 7 lessons *Investigation 4: 6 lessons 

Standards Addressed in the Unit  
2.OA.1 Use addition and subtraction within 100 to solve one and two step word problems involving situations of adding to, taking from, putting together, taking apart, and comparing, with unknowns in all positions, e.g. by using drawings and equations with a symbol for the unknown number to represent the problem. 2.OA.2 Fluently add and subtract within 20 using mental strategies. By end of Grade 2, know from memory all sums of two onedigit numbers. 2.OA.3 Determine whether a group of objects (up to 20) has an odd or even number of members, e.g., by pairing objects or counting them by 2s; write an equation to express an even number as a sum of two equal addends. 2.NBT.1 Understand that the three digits of a threedigit number represent amounts of hundreds, tens, and ones; e.g., 706 equals 7 hundreds, 0 tens, and 6 ones. Understand the following as special cases: 2.NBT.2 Count within 1000; skipcount by 5s, 10s, and 100s. 2.NBT.5 Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction. 2.NBT.6 Add up to four twodigit numbers using strategies based on place value and properties of operations. 2.NBT.9 Explain why addition and subtraction strategies work, using place value and the properties of operations 2.MD.8 Solve word problems involving dollar bills, quarters, dimes, nickels, and pennies, using $and ¢ symbols appropriately. Link to CCSS Unpacking Document 