Lesson objective: Understand that mental estimation strategies, other than rounding, may be more efficient when working with word problems.

Students bring prior knowledge of rounding from 3.NBT.A.1 and 3.OA.D.8. This prior knowledge is extended to include new estimation strategies such as compatible numbers, front-end estimation, and cluster estimation. A conceptual challenge students may encounter is understanding what mental computation strategy to use based on the problem given. Students using compatible numbers may struggle to recognize what numbers go together easily.

The concept is developed through work with money, which concretely represents the relationship between numbers and continues to help develop number sense to use in real-world situations.

This work helps students deepen their understanding of number because students work to determine patterns and relationships amongst numbers to make mental estimations. Students should recognize that some problems depending on the numbers work better with some numbers than others.

Students engage in Mathematical Practice 1 (Make sense of problems and persevere in solving them) as they make meaning from word problems and make decisions on how to go about solving them. As students arrive at a reasonable estimate, they should be able to ask themselves, "Does this make sense?" If not, they need to look for another solution pathway. Students also engage in Mathematical Practice 2 (Reason abstractly and quantitatively) as they attend to the meaning of the quantities and make decisions about what estimation strategy to use. Students should be allowed to use the mental computation strategy they are most comfortable with; however, it's important to give students ample opportunities to use a variety of estimation strategies and discuss the strategies they chose with each other.

**Key vocabulary:**

- compatible numbers
- efficient
- estimation
- justify
- strategy

**Special materials needed:**