Lesson objective: Understand that addition and subtraction can be used to compare measured or estimated lengths of objects.

Students bring prior knowledge of determining length differences through addition and subtraction from 2.MD.4 (Differences in length between to objects can be measured). This prior knowledge is extended to using operations as students find addition and subtraction problems to determine length differences. A conceptual challenge students may encounter is selecting the correct measurements and putting them in the correct order in the equations.

The concept is developed through work with a meter stick and tape diagrams, which illustrate both addition and subtraction.

This work helps students deepen their understanding of operations because it illustrates the inverse nature of addition and subtraction.

Students engage in Mathematical Practice 3 (Construct viable arguments and critique the reasoning of others) as they present and defend different solution paths.

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