The purpose of this activity is for students to decide if different estimates for a problem make sense, based on what they know about the problem type and place value. If they do not think an estimate makes sense, they are asked to show or explain a way to improve it. In the Activity Synthesis, they are encouraged to reflect on the strengths and weaknesses of each estimation strategy. As in the previous activity, a simple problem type is used to encourage students to focus on estimating rather than problem solving outside of 100.

• Invite 1 or 2 students to share their reasoning for Lin’s estimate.
• As needed, discuss how estimating can help make sure the right operation is used when solving a problem.
• Invite previously selected students to share their reasoning for Andre’s estimate.
• “We can estimate in many ways. How are Jada’s way and Andre’s way the same? How are they different?” (Both added the hundreds. Andre noticed the numbers were close to 225 and 375. He knew , and he added all the hundreds. Jada just added the hundreds.)
• “Sometimes Jada’s way of just adding (or subtracting) the hundreds can be a great way to estimate quickly if it’s not important to get close to the actual answer. Other times, it might be important to think of ways to quickly adjust your estimate so that it makes sense with the tens and the ones in the problem.”

The purpose of this activity is for students to use estimation strategies to reason about whether their computations make sense when adding and subtracting within 1,000. Monitor for the different ways students use what they know about place value and compatible numbers to reason about which estimates are too high, too low, or about right and their discussions with their partners about whether their answers make sense. Each problem includes at least one estimate that can be found using front-end estimation, without adjusting for the tens and the ones. Students are not incorrect when sorting these into the “about right” column, but consider looking for ways to pair them with students who place the estimates in other columns to compare their reasoning.

• Invite 1 or 2 groups to share their discussion for one of their expressions.
• “What was helpful about having someone look at your work for each problem?” (They thought my answer made sense, which helped me know that I had answered correctly. They caught a mistake that I made that I didn’t notice.)
• “What was helpful about looking at someone else’s work for each problem?” (I was able to see a different strategy that they used for the problem. I was able to help them catch a mistake they made.)