Lesson objective: Use cluster estimation to estimate sums and products when the numbers being added or multiplied cluster near or are close in value to a single number.

This lesson helps to build procedural skill with cluster estimation and help students determine which estimation strategies are most efficient for different contexts of problems. Bar models are used here because it helps students visualize the relationship amongst the quantities and models efficient problem solving with more complex tasks. This work develops students' understanding that in addition to rounding, we can use mental computation strategies to assess the reasonableness of responses.

Students engage in Mathematical Practice 1 (make sense of problems and persevere in solving them) as they explain to themselves the meaning of the problem and how to go about solving it. They should check their thinking and solutions by asking themselves, "Does this make sense?"

**Key vocabulary:**

- approximation
- cluster estimation
- reasonableness

**Special materials needed:**

- Smartboard, document camera, or overhead projector