The purpose of this activity is for students to solve two story problems involving adding or subtracting multiples of 10. Students are presented with a familiar context of counting cubes in different bags. The quantities of cubes are described using representations, and students add or subtract in a way that makes sense to them. The familiar context and representations are used to help students make sense of adding and subtracting tens. In the Activity Synthesis, students compare and connect the different representations used for each problem (MP2).

• Invite previously identified students to share their representations.
• “How are these representations alike? How are they different?”

The purpose of this activity is for students to practice adding and subtracting multiples of 10 from multiples of 10. Students may use any method that makes sense to them, such as using connecting cubes in towers of 10, making base-ten drawings, or adding or subtracting the single-digit number of tens. When students notice they can add tens to tens just like they add single-digit numbers, they look for and make use of the base-ten structure of numbers (MP7).

• “How did you find the number to make this equation true? How can you prove that the equation is true?”
• “How could you think about this equation as adding (or subtracting) tens?”

• Share responses and different methods used for each problem.