This lesson is optional because it is an opportunity for additional practice with estimating and with addition and subtraction within 1,000 that not all classes may need. Students also are invited to make sense of reasoning about a story problem that involves addition within 1,000.

In previous lessons, students used estimation strategies to decide if answers to one- and two-step story problems made sense. In this unit, students added and subtracted within 1,000, using a variety of different methods and representations, with an emphasis on place value.

In this lesson, students continue to develop their estimation strategies when problem solving, and use estimation strategies to determine whether their calculations make sense. As in previous lessons with estimation, students may use any strategy that makes sense to them. Some students may continue to use simple front-end estimation, while others may begin to consider how to make adjustments to their initial estimates, based on the values of the places. For example, when estimating the value of  , some students may add the hundreds and estimate 700, while others may start with 700 as the estimate and then adjust it to 800 because they can tell that will be at least 100 more. Both estimates are acceptable for grade 2 students, although by the end of the lesson, they should be able to explain why the latter estimate may be closer to the actual answer than the former.

Although the work of this lesson does encourage students to begin to consider precision in their estimates, it is important to continue to emphasize the reasoning students use to make their estimates, rather than how close an estimate is to an actual answer. Also, keep in mind that although students’ strategies may resemble “rounding,” they will not round numbers formally until grade 3.

Reflect on how you can reinforce the work done in today’s lesson outside of math class. When can you ask students questions involving estimation and assessing the reasonableness of answers? How can you encourage students to use this skill throughout the day when an actual answer is not needed?