Lesson objective: Understand that the numbers in an addition or subtraction problem determine the most efficient choice of solution strategy.

Students bring prior knowledge of addition and subtraction strategies from 3.NBT.A.1 and 3.NBTA.2. This prior knowledge is extended to justify strategy choices as students make decisions about the most efficient strategy to use when adding and subtracting. A conceptual challenge students may encounter is that the standard algorithms for addition and subtraction are always the most efficient choice.

The concept is developed through work with the standard algorithms for addition and subtraction and the decomposition to friendly numbers and mental math, which helps students to understand that the numbers and operations in a problem influence the choice of strategy to use.

This work helps students deepen their understanding of operations because they must examine the elements of the word problem and make decisions about the most efficent way to arrive at a solution.

Students engage in Mathematical Practice #2, reason abstractly and quantitatively as they make choices about strategies for solving various addition and subtraction problems based on the numbers within the problem. Students should examine the numbers and operations within a problem and determine if the standard algorithm is the most efficient strategy to use or if another strategy (mental math) would be more efficient. Students engage in Mathematical Practice #3, construct viable arguments and critique the reasoning of others as they give reasoning for their choice of solution strategy based on the numbers in the problem. This takes place as students discuss with one another their strategy choice and reasoning and also by participating in teacher-guided whole class discussions.

**Key vocabulary:**

- decompose
- justify
- standard algorithm
- strategy