# Lesson 2

Situaciones de suma y resta

## Warm-up: Observa y pregúntate: Dos tablas curiosas (10 minutes)

### Narrative

The purpose of this warm-up is to elicit observations about patterns in addition tables containing sums of two-digit addends that are multiples of 10. Each table is partially filled out to show certain behaviors of the sums and highlight some properties of operations. For example, the sums in the first table can illustrate the commutative property ($$10 + 30$$ and $$30 + 10$$ both give 40). The sums in the second table can help students to intuit the associative property ($$50 + 10 = (40 + 10) + 10 = 40 + (10 + 10) = 40 + 20$$, though students are not expected to generate equations as shown here).

While students may notice and wonder many things about the addition tables, focus the discussion on the patterns in the tables and possible explanations for them. When students make sense of patterns in sums and try to explain them in terms of the features of the addends and how they are added, they look for and make use of structure (MP7).

### Launch

• Groups of 2
• Display the tables.
• “¿Qué observan? ¿Qué se preguntan?” // “What do you notice? What do you wonder?”
• 1 minute: quiet think time

### Activity

• “Discutan con su pareja lo que pensaron” // “Discuss your thinking with your partner.”
• 1 minute: partner discussion
• Share and record responses.

### Student Facing

¿Qué observas? ¿Qué te preguntas?

+ 10 20 30 40 50
10 40
20 50
30 40 50 ? 70 80
40 70
50 80

+ 10 20 30 40 50
10 20 60
20 40 60
30 ?
40 60 80
50 60 100

### Activity Synthesis

• “¿Cómo creen que funcionan las tablas? ¿Cómo sabemos qué números van en las celdas?” // “How do you think the tables work? How do we know what numbers go in the cells?” (Each number in the row at the top is added to each number in the first column on the left.)
• For each of the following questions, give students a minute of quiet think time. Illustrate their responses with equations, if possible.
• “En la primera tabla, ¿por qué las sumas que hay en la fila de la mitad y en la columna de la mitad son el mismo conjunto de números?” // “In the first table, why are the sums in the middle row and the middle column the same set of numbers?” (The same pairs of numbers are added. The first number in the middle row and in the middle column are 40 because they are both the sum of 10 and 30, just added in different orders: $$10 + 30$$ and $$30 + 10$$.)
• “En la segunda tabla, ¿por qué las sumas desde la esquina izquierda de abajo hasta la esquina derecha de arriba son todas 60?” // “In the second table, why are the sums from the lower left corner to the upper right corner all 60?” (Each time, the first number being added goes up by 10 and the second number goes down by 10, so the sum stays the same.)

## Activity 1: Monumentos y cataratas (25 minutes)

### Narrative

The purpose of this activity is for students to solve word problems that involve adding or subtracting numbers within 1,000, using strategies they are familiar with from earlier grades. The goal is to elicit and highlight strategies that rely on place value understanding, in preparation for upcoming work on addition and subtraction algorithms, which also rely on place value.

Monitor for the following strategies as students work on the last problem about the Eiffel Tower:

• Starting at 328 and counting on by place to 674. This could be represented on a number line or a series of equations.
• Starting at 674 and counting back to 328. This could be represented on a number line or as a series of equations.
• Subtracting 328 from 674 using base-ten blocks, subtracting hundreds from hundreds, tens from tens, and ones from ones, trading a ten for more ones as needed.

As students interpret quantities in context, reason about ways to represent them, and consider the solutions in terms of the situation, they practice reasoning quantitatively and abstractly (MP2).

### Required Materials

Materials to Gather

### Launch

• Groups of 2
• “Vamos a resolver algunos problemas acerca de lugares famosos y monumentos muy altos” // “We are going to solve some problems that involve famous places and great heights.”
• “Tómense algunos minutos para leer los problemas y mirar las imágenes. Prepárense para compartir de qué creen que se tratan los problemas y en qué se parecen y en qué son diferentes” // “Take a couple of minutes to quietly read the problems and look at the pictures. Be prepared to share what you think the problems are about and how they are alike or different.”
• 2 minutes: quiet think time
• Share responses.
• Give students access to base-ten blocks.

### Activity

• “Con su pareja, resuelvan estos problemas” // “Work with your partner to solve these problems.”
• 5-7 minutes: partner work time
• Monitor for the strategies used to solve the Eiffel Tower problem and identify students who use different strategies to share during synthesis.
• As student work, consider asking:
• “¿Cómo pueden representar el problema?” // “How could you represent the problem?”
• “¿Qué representa esto en el problema?” // “What does this represent in the problem?”
• “¿Qué estrategias pueden usar para resolver el problema?” // “What strategies could you use to solve the problem?”

### Student Facing

Resuelve cada problema. Explica o muestra tu razonamiento.

1. Las cataratas de Iguazú, en Suramérica, marcan el límite entre Paraguay, Brasil y Argentina. Son las cataratas más grandes del mundo.

Las cataratas tienen dos partes. El agua cae 115 pies en la primera parte y 131 pies en la segunda parte. ¿Cuánto cae el agua en total?

2. En Washington, D.C., hay muchos monumentos que honran a personas importantes de la historia de los Estados Unidos.

El Monumento a Lincoln mide 99 pies de alto. El Monumento a Washington mide 555 pies de alto.

¿Cuánto más alto es el Monumento a Washington que el Monumento a Lincoln?

3. La Torre Eiffel de París, Francia, tiene 674 escalones que van desde el suelo hasta el segundo piso. Hay 328 escalones desde el suelo hasta el primer piso.

¿Cuántos escalones hay desde el primer piso hasta el segundo piso?

### Advancing Student Thinking

If students don't find a solution to the problems, consider asking:

• “¿De qué se trata este problema? ¿Qué se puede contar o medir en esta situación?” // “What is this problem about? What can be counted or measured in this situation?”
• “¿Cómo puedes representar el problema con bloques en base diez?” // “How could you represent the problem with base-ten blocks?”

### Activity Synthesis

• Select previously identified students to share in the sequence shown in the Student Responses.
• “¿En qué se parecen estas estrategias?” // “How are these strategies the same?”
• “¿En qué son diferentes?” // “How are these strategies different?”
• If no students mention using place value to find sums or differences, ask them about it.

## Activity 2: Diario sobre conexiones (10 minutes)

### Narrative

The purpose of this activity is for students to reflect on the strategies they used in the first activity. This is an opportunity to check in with students about the strategies from grade 2 they are comfortable using and those they find more challenging.

MLR8 Discussion Supports. Invite students to begin partner interactions by repeating the question, “¿Qué matemáticas hicieron hoy que se conectaban con algo que hicieron en un grado anterior? Describan algo que entiendan muy bien después de la lección de hoy” // “What math did you do today that connected to something you did in an earlier grade? Describe something you really understand after today’s lesson.”
Action and Expression: Develop Expression and Communication. Provide students with alternatives to writing on paper: students can share their response to the prompt orally, with the option of using manipulatives, instead of writing it on paper.
Supports accessibility for: Fine Motor Skills, Social-Emotional Functioning

• Groups of 2

### Activity

• “Tómense un momento para responder alguno de estos temas de diario. Pueden responder más de uno si tienen tiempo” // “Take some time to respond to one of these journal prompts. You can respond to more than one prompt if you have time.”
• 5-7 minutes: independent work time
• “Ahora compartan su respuesta con su pareja durante unos minutos” // “Now, take a few minutes and share your response with your partner.”
• 2 minutes: partner discussion

### Student Facing

Responde alguno de estos temas de diario:

• ¿Qué matemáticas hiciste hoy que pudiste conectar con algo que hiciste en un grado anterior?
• Describe algo que entiendas muy bien después de la lección de hoy.
• Describe algo que haya sido confuso o retador, o acerca de lo que quisieras aprender más.

### Activity Synthesis

• Invite 2-3 students to share their journal responses with the class.

## Lesson Synthesis

### Lesson Synthesis

“Hoy usamos varias estrategias para resolver problemas de sumas y restas hasta 1,000” // “Today we used different strategies to solve problems that involve addition and subtraction within 1,000.”

“¿Qué estrategia les gustaría usar para sumar y restar, y por qué?” // “What is a strategy you like to use for addition or subtraction and why?” (I like to use base-ten blocks to subtract because it helps me see when I need to trade for more ones.)

“¿Sobre qué estrategia para sumar o para restar les gustaría aprender más?” // “What is a strategy for addition or subtraction that you would like to learn more about?” (I would like to learn more about the counting up strategy that can be used in subtraction problems.)