The purpose of this activity is for students to use a number line to compare different methods for getting to a ten when subtracting a two-digit number from a two-digit number. Students analyze and try a method where they add or subtract the tens first and then decompose the ones to reach a multiple of 10. Diego’s method is an example of this for .

Students also analyze a method that begins by decomposing the ones being added or subtracted to get to a multiple of ten. Tyler’s method is an example of this for .

In the Synthesis, students discuss each method and explain which one they prefer.

• Invite students to share how they used Diego’s or Tyler’s methods.
• “¿Qué método encontraron más útil, el de Tyler o el de Diego? Expliquen” // “Did you find Tyler’s method or Diego’s method more helpful? Explain.”
• “¿Hay otra forma de usar la recta numérica para mostrar cómo encontrar más fácilmente el valor de   completando una decena?” // “Is there another way we could use the number line to show a way to make it easier to find the value of by getting to a ten?”
• If not suggested by students, ask: “¿Cómo podemos empezar con 29 y usar un método como el de Diego o el de Tyler?” // “How could we start with 29 and use a method like Diego’s or Tyler’s?” (Start with 29, jump right 1 to get to 30, then it’s easy to see all you need to do is jump right 23 to get to 53.)
• Record the method and display throughout the lesson for students to reference.
• “¿En qué se parece este método al método de Diego o al de Tyler? ¿En qué es diferente?” // “How is this method the same as Diego's or Tyler’s? How is it different?”

The purpose of this activity is for students to continue to develop fluency with addition and subtraction within 100. The numbers in each expression encourage the use of the methods students analyzed in the previous activity. However, students should be encouraged to use whatever method makes the most sense to them. Partners work together to create a visual display to share their representations, including number lines, of one sum and one difference and do a Gallery Walk to compare their representations to others (MP2).

This activity uses MLR7 Compare and Connect. Advances: representing, conversing

• “Con su compañero, hagan una representación visual que muestre cómo pensaron para encontrar el valor de una suma y de una diferencia” // “Create a visual display that shows your thinking for one sum and one difference with a partner.”
• “Deben incluir una recta numérica para por lo menos una expresión de su presentación. Incluyan diagramas en base diez u otros detalles para ayudarle a otros a entender cómo pensaron” // “You should include a number line for at least one expression in your display. You may want to include base-ten diagrams or other details to help others understand your thinking.”
• 5 minutes: partner work time
• 6 minutes: Gallery Walk 
• “¿En qué se parecen y en qué son diferentes las representaciones?” // ”What is the same and what is different between the representations?”