The purpose of this activity is for students to revisit what it means for an answer to make sense. They see that they can use what they know about place value, combinations of ten, and friendly numbers to estimate and determine if an answer makes sense.

The prices chosen for the activity are used to encourage students not only to use front-end estimation strategies, but also to consider ways they might use compatible (“friendly”) numbers to estimate. For example, students might use what they have learned about quarters to use 25 as a friendly number when estimating the number of notebooks that Noah is able to buy. The context of the task prompts students to estimate. If students begin computing the exact numbers of beads, remind them of the situation and that they do not need to solve to determine if the answer makes sense.

As students work, prompt them to explain their strategies for making estimates. Listen for the ways that students begin to reason about the consequences of their estimation strategies, how they consider what to do with the value of the digits they do not add , and how they are changed when they use a compatible number. When students share their estimate and reason about how close their estimate is to the actual value, they attend to and communicate with precision (MP6). As in previous lessons, do not emphasize student responses that result in the most precise estimates. Encourage students to try a variety of strategies and to explain their reasoning.

• Invite students to share their responses for each set of items.
• Consider asking, “Did anyone estimate in a different way?”
• “When we are deciding whether an answer makes sense, we are not solving the problem to get an exact answer. We are estimating the answer.”
• “We can use what we know about what is happening in the problem and what we know about tens, ones, and friendly numbers to help us estimate.”

The purpose of this activity is for students to solve two-step word problems involving addition and subtraction. After students solve the problems, they trade answers with a partner to decide if their answer makes sense.

When students assess the reasonableness of each other’s answers and communicate their assessment, they construct logical arguments and critique the reasoning of others (MP3).

• “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 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.)