Lesson objective: Determine whether a product is reasonable using estimation and knowledge of place value.

Students bring prior knowledge of multiplication from 4.NBT.B.5. This prior knowledge is extended to determining reasonableness as students use estimation to predict a reasonable product using the standard algorithm. A conceptual challenge students may encounter is thinking that digit values are not important when calculating using the standard algorithm, or that you simply multiply digits and not their values.

The concept is developed through work with estimating partial products and a total product, which students can then use to monitor their thinking during multiplication to ensure they are multiplying digit values. The total product estimate can be used to determine the reasonableness of a final product.

This work helps students deepen their understanding of place value and its role in the standard algorithm because multiplying digit values is essential to successful use of the standard algorithm.

Students engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) as they continue to use the standard algorithm and reason around the reasonableness of their results. Students should recognize when digit values have not been multiplied correctly and when the algorithm has been applied incorrectly.

**Key vocabulary:**

- algorithm
- digit value
- distributive property
- estimation
- multiplication
- partial product
- place value
- product
- standard algorithm