# Lesson 3

Sumemos, expliquemos

## Warm-up: Conversación numérica: Sumemos más decenas (10 minutes)

### Narrative

The purpose of this Number Talk is to elicit strategies and understandings students have for adding within 100. These understandings help students develop fluency and will be helpful later in this lesson when students add two-digit numbers using methods based on place value.

### Launch

• Display one expression.
• “Hagan una señal cuando tengan una respuesta y puedan explicar cómo la obtuvieron” // “Give me a signal when you have an answer and can explain how you got it.”
• 1 minute: quiet think time

### Activity

• Keep expressions and work displayed.
• Repeat with each expression.

### Student Facing

Encuentra mentalmente el valor de cada expresión.

• $$13 + 21$$
• $$13 + 31$$
• $$23 + 31$$
• $$33 + 41$$

### Activity Synthesis

• “¿De qué forma saber el valor de $$23 + 31$$ puede ayudarlos a encontrar el valor de $$33 + 41$$?” // “How can $$23 + 31$$ help you solve $$33 + 41$$?” (The tens are changing again. In $$33 + 41$$, you are adding 2 tens total, so the sum of $$23 + 31$$ increases by 20. $$54 + 20 = 74$$.)

## Activity 1: Lin y Han suman (15 minutes)

### Narrative

The purpose of this activity is for students to make sense of the first step of addition methods and then finish finding the sum. They write equations to represent their thinking. In the synthesis, students have opportunities to share how they represented a method with equations and connect the representations using their understanding of place value (MP7).

MLR7 Compare and Connect. Synthesis: After all students' work has been presented, lead a discussion comparing, contrasting, and connecting the different approaches. Ask, “¿En qué se parecen y en qué son diferentes las estrategias de Lin y de Han?” // “How are Lin and Han’s approaches similar and different?”

### Launch

• Groups of 2
• Give students access to connecting cubes in towers of 10 and singles.

### Activity

• 7 minutes: partner work time
• Monitor for a student who writes equations to represent how each student can find the value and can explain their thinking.

### Student Facing

Lin y Han están encontrando el valor de $$32+ 54$$.
¿Qué deben hacer después para encontrar el valor?
Escribe ecuaciones para representar cómo pensaste.
Prepárate para explicar cómo pensaste de una forma que los demás entiendan.

1. Lin empezó juntando 3 decenas y 5 decenas.
Ella anotó su primer paso así: $$30 + 50 = 80$$.

2. Han empezó con 54 y después sumó 3 decenas.
Él anotó su primer paso así: $$54+ 30 = 84$$.

### Student Response

If students take new steps that do not lead to a value of 86, consider asking:

• “¿Qué piensas que Lin (o Han) debe hacer después? ¿Por qué?” // “What do you think Lin (or Han) should do next? Why?”
• “¿Qué hizo Lin (o Han) primero? ¿Cómo podrías usar cubos encajables para mostrar lo que hicieron?” // “What did Lin (or Han) do first? How could you show what they did with connecting cubes?”
• “¿Qué tienen que hacer después? ¿Cómo lo sabes?” // “What do they need to do next? How do you know?”
• “¿Cómo podrías representar el siguiente paso con una ecuación?” // “How could you represent the next step with an equation?”

### Activity Synthesis

• Invite previously identified students to share their equations for Lin's work and then Han's work.
• “¿En qué se parecen estas ecuaciones? ¿En qué son diferentes?” // “How are these equations the same? How are they different?” (They both got the same sum. They both add tens and ones. The first one breaks apart both numbers and adds tens and tens and ones and ones. The second one starts with 54 and adds the tens then the ones.)

## Activity 2: Te escuché decir (20 minutes)

### Narrative

The purpose of this activity is for students to find the sum of 2 two-digit numbers and represent their thinking with equations. Students then orally explain their steps to a partner and how the equations they wrote match their steps. Partners listen and then practice restating what they heard their partner say. Restating what a partner has said helps ensure students listen for understanding as their partner explains their thinking. Students have multiple opportunities to explain their thinking and the thinking of a partner and revise their language for clarity (MP3, MP6).

This activity uses MLR8 Discussion Supports. Advances: listening, speaking, conversing.

Action and Expression: Internalize Executive Functions. Invite students to verbalize the method they will use before they begin. Students can speak quietly to themselves or share with a partner.
Supports accessibility for: Organization, Conceptual Processing, Language

### Launch

• Groups of 2
• Give students access to connecting cubes in towers of 10 and singles.

### Activity

• 5 minutes: independent work time
• Monitor for students using different methods in order to make partnerships of students who found the value in different ways.
• “Ahora van a explicarle a su pareja cómo encontraron el valor de la suma. Usen sus dibujos o cubos encajables para explicar sus pasos. También explíquenle cómo las ecuaciones que escribieron corresponden a cada paso que siguieron” // “Now you are going to explain to your partner how you found the value of the sum. Use your drawings or connecting cubes to explain your steps. Also explain how the equations you wrote match each step you took.”

MLR8 Discussion Supports

• “Después de que su pareja explique su método, digan en otras palabras lo que escucharon” // “After your partner explains their method, restate what you heard from your partner.”
• Display the sentence frame: “Te escuché decir . . .” // “I heard you say . . .”
• “Pregúntenle a su compañero si expresaron correctamente sus ideas” // “Ask your partner if you restated their thinking accurately.“
• 8 minute: partner discussion

### Student Facing

Encuentra el valor de $$23 + 74$$ usando el método de Lin o el método de Han.
Escribe ecuaciones para representar tus ideas.

$$23 + 74$$

### Activity Synthesis

• “¿Qué les pareció lo más difícil al intentar expresar de otra manera las ideas de su pareja? ¿Qué les ayudó a entender las ideas de su pareja y a decirlas con otras palabras?” // “What was most difficult about restating your partner’s thinking? What helped you make sense of your partner’s thinking and restate it?”

## Lesson Synthesis

### Lesson Synthesis

Display:

31 + 48
48 + 30 = 78
78 + 1 = 79

“Hoy encontramos sumas de 2 números de dos dígitos, explicamos cómo pensamos y practicamos cómo decir con otras palabras lo que nuestra pareja pensó. Escuchen mientras explico cómo encontré el valor de 31 + 48. Después, tendrán la oportunidad de decir con otras palabras lo que me escucharon decir” // “Today we found the sums of 2 two-digit numbers, explained our thinking, and practiced restating our partner's thinking. Listen as I explain how I found the value of 31 + 48. Then you will have a chance to restate what you heard me say.”

Explain the steps represented by the equations and invite students to share beginning with, “Te escuché decir . . .” // “I heard you say...”

## Student Section Summary

### Student Facing

Hoy sumamos números hasta 100. Aprendimos diferentes métodos para sumar pensando en decenas y unidades, y usamos ecuaciones para mostrar cómo pensamos.

$$23 + 45$$

Sumar las decenas y después las unidades:

$$23 + 40 = 63$$
$$63 + 5 = 68$$

$$20 + 40 = 60$$
$$3 + 5 = 8$$
$$60 + 8 = 68$$