1. Think Addition Unit of Study 5: Modeling and Representing 2-Digit Addition and Subtraction Global Concept Guide: 4 of 5

2. Content Development  The purpose of this GCG is for students to think of addition to solve subtraction problems.  Many students think of inverse operations when solving basic facts under 20, but fail to use this strategy outside of this context.  E.g. when solving 13-8=? Many students will think of 8 + ?= 13. However, when solving 72-48, students see this as an abstract concept, focused solely on subtraction.  The strategy of think addition is best utilized when using one hundred charts and open number lines. Once students can visualize a number line, many students can solve most two digit problems mentally.

3. Day 1  Today is the day to break out 100 charts again! The more you use 100 charts, the faster students are able to visualize the numbers and can then mentally solve 2-digit subtraction problems more efficiently!  When solving 73-52, students need to think addition (52+?=73)  Now using the number sentence: 52+__=73 Possible student response: I can start at 52, add ten more then add ten more to get to 72 then add one more to get to 73. So 10+10+1=21. So 73-52=21. Possible student response: I can start at 52, add ten more then add ten more to get to 72 then add one more to get to 73. So 10+10+1=21. So 73-52=21. Key =subtract 10 =add 10 =add 1 = subtract 1

4. Day 2  The focus today should be on using open number lines for the “think addition” strategy  Students should be given multiple problems to practice using open number lines.  Remember the most important part about using an open number line is for students to track their thinking however they wish.  It would be great for students to think critically about the numbers they are working with. In order to do this, they need to discover the most efficient ways to jump on the open number line by working with their peers and listening to different ways people solved the problem.

5. Day 2 Open Number Line

6. Enrich/Reteach/Intervention  Arrow math, as seen on Day 1 can be an extremely useful tool in both reteaching and enriching students.  In order to enrich students, you can give possible tasks such as:  How can you show regrouping in a subtraction sentence using arrow math?  Without using a 100 chart, show the arrows you would draw to use the strategy of think addition for the following problem: (45-16)