The purpose of this activity is for students to add two-digit numbers and three-digit numbers that require composing a ten when adding by place. In this activity, students work in groups of 3. Within their group, each student finds the values of a different set of sums. They may use any method of addition that makes sense to them. The number choices in each set encourage students to apply methods they have used when adding within 100, including those based on making a ten, counting on by place, and adding tens to tens and ones to ones. In the Activity Synthesis, students share the patterns they notice in the sums and their values. Look for opportunities to highlight the ways students notice that the sum of the ones is 10 in each expression, to prepare them for the next activity.

If students find a sum other than the sum of the two numbers, consider asking:

• “¿Puedes explicar cómo encontraste la suma?” // “Can you explain how you found the sum?”
• “¿Cómo puedes usar una recta numérica o bloques en base diez para mostrar cómo pensaste?” // “How could you use a number line or base-ten blocks to show your thinking?”

• “¿Qué patrones observaron en cada conjunto?” // “What patterns did you notice in each set?” (Each time we could make a ten with the ones. The number of hundreds didn't change. The sums went up by 10.)
• Share and record responses.
• “Saben que  . ¿Cómo les ayuda eso a pensar en  ?” // “How does knowing help you think about ?” (I can add . Then add 10, because I know .)

The purpose of this activity is for students to use what they know about combinations of 10 to identify when a ten will be composed when adding a three-digit number and a two-digit number by place. Students are given a set of cards with three-digit numbers and two-digit numbers, and they work with their group to decide which numbers will make a ten, with no extra ones, when they are added together (a “perfect ten”). After finding all the matches, each group member chooses a pair and finds the value of the sum. In the Activity Synthesis, students discuss how they could tell a pair of numbers would make a ten by looking at the digits in the ones place. In upcoming lessons, students will use this understanding to anticipate when they may need to compose units when they add 2 three-digit numbers.

When they match numbers whose ones combine to make a ten, students look for and make use of structure, which can be helpful when calculating sums (MP7).

• Invite previously selected groups to share.
• Attend to the language that students use to describe their matches, giving them opportunities to describe how they knew adding the numbers would result in composing a ten, with no extra ones, more precisely.