The purpose of this activity is for students to make sense of a strategy used to solve an Add To, Change Unknown story problem. The representation shows counting on from the known addend to the known result (total). Students make sense of the representation and interpret how it shows the quantities in the problem, including the answer. In the Synthesis, students make sense of using to represent an unknown in an equation. Students will continue to make sense of equations throughout the unit, but they are not expected to produce them on their own until the end of the unit.

Students keep their books closed for the Launch of the activity, as the teacher displays and reads the problem.

• “¿Cómo el dibujo de Lin muestra el problema-historia?” // “How does Lin’s drawing show the story problem?” (She shows the 3 books Andre starts with, and the 6 books she needs to get to 9.)
• Display: 3 + something is 9.
• “Lin comenzó en 3 y contó hasta llegar a 9. ¿9 es la respuesta al problema? ¿Por qué sí o por qué no?” // “Lin started at 3 and counted more until she got to 9. Is 9 the answer to the problem? Why or why not?” (No. 9 is the total. The answer is how many she added to 3 to get 9.)
• “Podemos mostrar cómo piensa Lin usando una ecuación con un sumando desconocido” // “We can show Lin’s thinking using an equation with an unknown addend.”
• Display:
• “¿Qué relación hay entre la ecuación y la afirmación ‘3 más algo es 9’?” // “How does this equation match the statement ‘3 plus something is 9’?” (It’s the same except there’s a box instead of “something” and an equal sign instead of “is”)
• “El cuadro representa el número que podemos sumarle a 3 para que el valor sea 9. El cuadro representa el sumando desconocido. ¿Qué número escribiríamos en el cuadro?” // “The box represents the number that we could add to 3 so that the value is the same as 9. The box represents the unknown addend. What number would we write in the box?” (6)
• Write 6 in the box.
• “Esta ecuación es verdadera porque sabemos que   es lo mismo que, o es igual a, 9.   es igual a 9” // “This equation is true because we know is the same as, or equal to, 9. equals 9.”

The purpose of this activity is for students to continue to make sense of equations with a symbol for an unknown addend and to compare strategies for determining the number that makes an equation true. Students continue to share and compare addition strategies, including those based on counting on.

Although it is recommended to focus on counting on strategies in the Lesson Synthesis, monitor for other strategies students may use. For example, some students may represent the total and separate the known addend, count back from the total to the known addend, or use a known fact.

• Invite 2–3 previously selected students to share how they found the unknown addend.
• “¿En qué se parecen sus métodos? ¿En qué son diferentes?” // “How are their methods the same? How are they different?”
• “¿De qué otras maneras descifraron cómo hacer que las ecuaciones sean verdaderas?” // “What were some other ways you figured out how to make the equations true?”