In previous lessons, students solved different types of story problems within 100. They used diagrams and equations to make sense of and solve story problems. In this lesson, students build on their strategies for making sense of a problem by considering ways they might estimate an answer before finding the actual answer. Although students may informally round numbers, the concept and skill of rounding numbers is not expected of students until grade 3. At this point, students may use any strategy that makes sense to them to justify why an estimate is or is not reasonable. This may include simple front-end estimation (for example, students describe their strategy as just adding the tens in and say it is about 70). Others may think about replacing numbers with friendly numbers (for example, they may say is about or 80).

Throughout the lesson, it is important to emphasize the ways students reason about the reasonableness of estimates and not how close an estimate is to the actual answer. Emphasizing the latter can frustrate students and encourage them to perform the actual calculation first before sharing their “estimate.”