The purpose of this activity is for students to consider what it means for an answer to make sense. They see that they can use what they know about place value and strategies, such as finding friendly numbers, to estimate the answer to a problem and determine if an answer makes sense.

The quantities chosen are close to multiples of 10 to encourage students to consider strategies like front-end estimation (for example, in this task, adding or subtracting just the tens place). If students begin computing the exact numbers of seeds, remind them of the situation and language of the problem. For example, the first problem says that “Mai makes an estimate” and “about 80 seeds.” Also remind students 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 and relate those strategies to the ways students have reasoned about place value or methods they have used to add and subtract, such as looking for friendly numbers (MP3). When students use language, such as “about 80 seeds” to convey that they are estimating, they practice communicating with precision (MP6).

• “How did you decide whether the estimate was reasonable?”
• “How could you decide without adding (or subtracting) the tens and the ones?”

• Invite previously identified students to share their responses to the problem with Kiran’s seeds.
• As needed, consider asking:
• “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. How are 60 seeds and 50 seeds both reasonable estimates for the number of seeds?” (60 seeds makes sense because you just think about 8 tens take away 2 tens. 50 seeds also makes sense because 28 is really close to 30.)
• “Why is 100 not a reasonable estimate?” (The story is about taking away seeds, not adding more. 100 is not close to 8 tens minus 2 tens.)
• “In general, how can using what we know about place value help us estimate?” (Just adding or subtracting the tens makes the numbers easier to think about quickly. The real answer might be more or less, but it can help think about an answer that makes sense.)

The purpose of this activity is for students to determine if different estimates make sense and to make their own estimates for different problem types. In order to help students focus on reasoning about estimation, the activity uses simple Add To and Take From problem types. Students are asked to reason about an estimate and create their own estimates for a simple two-step problem. The actual answers to these problems are not shared, and it is not recommended that you have students find the actual answers to avoid students focusing on trying to find the “closest estimate” at this point. Help students focus on justifying and making reasonable estimates based on place value.

• Invite students to share their too high, too low, and about right estimates for the problem with Diego’s seeds.
• Consider asking:
  • “How do you know this number would be too high or too low?”
  • “Why does your about right estimate make sense for this problem?”
• If multiple “about right” answers are shared, invite students to share how they made their estimates and compare the different estimation strategies. Remember to avoid praising strategies that result in more precise estimates at this point or comparing how close each is to the actual answer.