Lesson 16

The purpose of this warm-up is for students to interpret an image of a shopping cart full of groceries. Students reflect on what they usually shop for. This will be useful when they investigate the market context further during the activities.

• Groups of 2
• Display the image.
• “Han went grocery shopping with his family. Here is their basket. What do you notice? What do you wonder?”
• 1 minute: quiet think time

• “Discuss your thinking with your partner.”
• 1 minute: partner discussion
• Share and record responses. 

• “What are some things you or your family buy at the grocery store?”
• “Where are other places we can buy things?” (online, markets like bodegas or delis, convenience stores, supermarkets, big box stores)

In this activity, students decide on the inventory for items sold in a market of their choice. They must keep 3 kinds of items in stock and use their understanding of adding and subtracting within 100 to make sure they have a total of 100 items in their store. They will use their completed inventory sheet in the next activity.

• Groups of 2.
• “You will decide 3 kinds of items to sell in a store of your choice and have a total of 100 items.”
• “When you write how much you have at the beginning of the day, for one of the items, you should have less than 10 in total.”
• “You should have more than 10 each of the other two items you pick.”

• “In this activity, you will fill out only the first two rows. Then complete the rest in the next activity.”
• 5 minutes: independent work time.
• 2 minutes: partner discussion

• “What are some ways we can prove that we have exactly 100 items?“ (Add up the three numbers and get 100. Subtract two numbers from 100 and get the last number. Add two numbers and then subtract the result from 100 to get the third number.)

In this activity, students role-play running a market and shopping at each other’s stores. Students use their understanding of adding and subtracting within 100 to sell and restock their items using their inventory sheet. Students roll a number cube and use the results to decide how much of each item they buy in the store. Throughout the activity, they must make sense of the numbers and operations they use in the context of the market scenario (MP2). Buyers must use their results strategically and interpret them in the context of the number of items available at the store before they buy. Sellers must also keep track of their inventory by keeping up with how many of an item has sold and comparing that with how many they have in stock.

As an alternative to students going around the room to buy and sell, students can work in groups of 4.

• Groups of 2
• Give each group a number cube.
• Read the first problem.
• Students roll and record.
• Read the directions for buyers.
• As needed, demonstrate how students can:
  • use the results of the rolls as digits to create two-digit numbers
  • add or subtract the values of their rolls to create new two-digit or one-digit numbers
• Read the directions for sellers and show where they will update how many items they have sold.
• “Use the numbers you rolled to decide how much of something you will buy. Before you make your decisions, look at the inventory. Consider your possible numbers.”
• 2–3 minutes: partner work time

• “Pair up with another person in the room. Take turns and buy from each other. Use the digits from your rolls to decide how much of an item you can buy.”  
• “If you are selling, update your ‘sales activity’ after each purchase. Before a new buyer makes a purchase, make sure you have enough in your inventory.”
• “When you finish buying and selling, move on to another person until I tell you to stop.”
• 10–15 minutes: partner work time
• Monitor for students who:
  • strategically use the results of the rolls as digits or add or subtract their results before they buy
  • compose or decompose tens when they update their sales or consider if they have enough of an item to sell
• If opting to not complete the next activity: When students return to their seats, “Record your end-of-day total. Summarize what happened in your store to your group.”

• Invite previously selected students to share.
• “How did you decide how to use rolls strategically, based on what was in the store’s inventory?” (If there were more items, I used a two-digit number. I had to think about which digit goes first.)
• “Did anyone run low on an item? Did anyone run out of an item? What did you tell your buyer?”

In this activity, students summarize the sales activity in their store by using their inventory sheet to record their total sales, their ending inventory, and the number of items they need to restock the shelves for the next day. They use their understanding of the relationship between addition and subtraction to observe that the number of items subtracted (total sales) is the same as the number of items that need to be added to get back to the starting inventory (MP7).

Students give a sales report to summarize the activity in their store.

• Same groups of 2
• “When you work in a store, at the end of the day you have to see how many items you have sold and you have to restock your shelves for the next day.”

• “For each product, record how many items were sold, how many are left, and how many are needed to restock.”
• 5 minutes: independent work time
• “Now share your sales report with your partner.”
• 4 minutes: partner discussion
• Monitor for:
  • students who share a clear and organized presentation
  • students who notice the restock amount and the sale amount is the same

• Invite previously identified students to share their sales reports.
• “How did you decide the number of items to get from the stockroom?”
• “How can we be sure this is the correct restock amount?” (We’re replacing the exact amount that was sold. If we add the restock amount and the number of items left, we get the original starting amount.)