# Teacher Tool of the Month: Build Expressions Equal to Twenty Build Expressions Equal to Twenty The Snap Blocks™ tool is an engaging way for students to make sense of the concept of equality and begin to make generalizations that will inform their algebraic thinking. Students will understand the meaning of the equal sign and the concept of equivalence in this math lesson plan. They will explore relational thinking by noting relationships between two sides of an equation.

## Sample Lesson Plan

 Objective: Students are able to prove that two expressions are equivalent by recognizing smaller parts of the expressions that are equivalent. Background: Students should have an understanding of basic addition number facts within 20. Instruction: Introduce the problem to students. “We have an equation posted on the board.  Our task is to find out if these two expressions are equivalent.” Ask students, “If we don’t know the answer off the top our head, does anyone have a strategy?”  Possible responses: “We can use the snap blocks on the bottom to represent the problem and test it.” “We can put the green blocks on the tray and then see if any of the combinations of the blue blocks line-up with the green blocks.” “We can add up both sides to see if they have the same totals.” “We can see if some of the green numbers make groups of tens.” Ask another student to come to the board and prove if the equations are equal using the snap blocks to represent the expressions. Allow the student to experiment with the blocks and provide an answer. It’s OK if students provide an incorrect response. This gives the class an opportunity to discuss whether the student’s strategy and conclusions are valid in the next step. Once the student has made her case, ask the class to vote on whether the statement is true or false.  As another student to come to the board and press the true or false button according to the class decision. If the response is incorrect, ask another student to come up to the board and explain why the statement is true or false.  Allow students to make mistakes and defend their decisions.  The purpose is to not only be able to answer the prompt but to understand why the statement is true or false and be able to defend their responses.   If the response is correct, click the “Next” button and begin a new problem.  Engage all students in solving problems and ask them to construct and argument, explain their thinking, and justify their answers.  The other students in the class can verbally critique these arguments and explanations. Use questions to scaffold as needed.

## Common Core State Standards

 Grade ID Domain Cluster Standard 2 2.NBT.5 Number and Operations in Base Ten Use place value understanding and properties of operations to add and subtract. Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction. 2 2.OA.1 Operations & Algebraic Thinking Represent and solve problems involving addition and subtraction. 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 2.OA.2 Operations & Algebraic Thinking Add and subtract within 20 Fluently add and subtract within 20 using mental strategies. By end of grade 2, know from memory all sums of two one-digit numbers. 2 2.SMP.2 Mathematical Practices Reason abstractly and quantitatively.