The purpose of this activity is for students to continue to make sense of different ways to solve Compare, Difference Unknown problems. In this activity, the unknown difference is greater than in previous lessons. Connecting cube towers are used to allow students to compare different ways of determining the difference and to make connections to the operations of addition and subtraction.

• Invite previously identified students to share how they found how many fewer cubes Diego had.
• Annotate strategies that show adding on cubes to the smaller quantity or counting without explicitly removing blocks with .
• Annotate strategies that show removing cubes (physically or by crossing out) and counting what’s left with .
• “What do you notice about all these methods?” (They all got the same answer, even if they added cubes or took cubes off.)
• “What goes in the box for each of these equations?” (7)
• “The cubes help us show that we can think of Compare problems as an unknown addend problem.”
• Demonstrate a students strategy with cubes by holding up a tower of 3 and a tower of 10.
• “I can think of finding the difference as figuring out how many I would have to add to 3 to be the same as 10.”
• Demonstrate counting on while adding 1 cube at a time to the smaller tower.
• “The cubes can also help us show that subtraction is another way to find an unknown addend.”
• Demonstrate matching the towers and removing 3 cubes as you count back.
• “How is what I just did like ?”
• “How does represent the bigger tower, the smaller tower, and the difference?”

The purpose of this activity is for students to solve Compare, Difference Unknown story problems in the context of connecting cube towers. Students solve comparison problems with given constraints that encourage students to add or break apart cubes to make towers with the same number of cubes. As students explain their thinking during the Activity Synthesis, record both addition and subtraction equations. When students answer the question, "How do you know?" they are beginning to explain their reasoning and construct viable arguments (MP3).

• Invite 1–2 groups to share the different ways they found the answer to the last problem.
• “What was the same about both ways of solving the problem? What was different?”
• “Which way did you like best? Why?”