In this activity, students use given descriptions and their knowledge of multiplication to make some estimates of the cost and the time associated with brushing teeth. To complete the estimation, students need to make some assumptions about what constitutes “a lifetime” and the cost of a single toothbrush. The activity gives students another example before they write their own Estimation Exploration.

• Select students, who made different assumptions about cost and the length of a lifetime, to share their responses and reasoning. Record their estimates.
• If time permits, invite students to share the new estimation questions they thought of for this situation. “¿Cómo podrían hacer esas estimaciones?” // “How might you make those estimates?”

In this activity, students create an Estimation Exploration that focuses on multi-digit multiplication. Students may choose to present a situation (as in the toothbrush example), something observed in the classroom (such as a large shelf filled with books), or an image (as in the Warm-up). They need to anticipate the reasoning strategies others might use to make an estimate.

• Encourage students to ask lingering questions about designing their activity.
• Clear up any confusion and allow them a few minutes to revise their design, if needed.

In this activity, students facilitate the Estimation Exploration they created for another group in the class. If time allows, encourage students to run the activity for multiple groups.

• See the Lesson Synthesis.