Lesson 4

Más suma y resta con decenas

Warm-up: Conversación numérica: Más o menos 10 (10 minutes)

Narrative

The purpose of this Number Talk is to elicit strategies and understandings students have for adding and subtracting 10. These understandings help students develop fluency and will be helpful later in this lesson when students add or subtract multiples of 10.

When students share strategies based on adding or subtracting the digits in the tens place and explain why this method works, they are looking for and making use of the base-ten system and expressing regularity in repeated reasoning (MP7, MP8).

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.

• $$30 + 10$$
• $$40 + 10$$
• $$80 - 10$$
• $$70 - 10$$

Activity Synthesis

• “¿Alguien usó el mismo método pero lo explicaría de otra forma?” // “Did anyone use the same method but would explain it differently?”
• “¿Alguien pensó en el problema de otra forma?” // “Did anyone approach the problem in a different way?”

Activity 1: ¿Cuántos hay en la bolsa? (20 minutes)

Narrative

The purpose of this activity is for students to solve two story problems involving adding or subtracting multiples of 10. Students are presented with a familiar context from a previous lesson in which students counted cubes in different bags. The quantities of cubes are described using representations that students are familiar with, and students add or subtract in a way that makes sense to them. The familiar context and representations are used to help students make sense of adding and subtracting tens. In the synthesis, students compare and connect the different representations used for each problem (MP2).

MLR6 Three Reads. Keep books or devices closed. To launch this activity, display only the problem stem, without revealing the question. “Vamos a leer este problema-historia tres veces” // “We are going to read this story problem three times.” After the 1st Read: “Cuéntenle a su compañero lo que ocurrió en la historia” // “Tell your partner what happened in the story.” After the 2nd Read: “¿Cuáles son todas las cosas de esta historia que podemos contar?” // “What are all the things we can count in this story?” Reveal the question. After the 3rd Read: “¿De qué formas diferentes podemos resolver este problema?” // “What are different ways we can solve this problem?”

Required Materials

Materials to Gather

Launch

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

Activity

• 7 minutes: independent work time
• 3 minutes: partner discussion
• Monitor for students who show:
• towers of 10
• base-ten drawings
• __ tens and __ tens
• expressions or equations

Student Facing

1. Jada está contando colecciones de cubos.
En la bolsa A hay 30 cubos.
En la bolsa B hay 2 torres de 10.
¿Cuántos cubos hay en total en las dos bolsas?
Muestra cómo pensaste. Usa dibujos, números o palabras.

2. Tyler está contando una colección de cubos.
En la bolsa C hay 7 torres de 10.
Él saca 40 cubos de la bolsa.
¿Cuántos cubos le quedan en la bolsa?
Muestra cómo pensaste. Usa dibujos, números o palabras.

Student Response

If students attempt to add or subtract using single cubes (or representations of single cubes), consider asking:

• “¿Cómo planeas descubrir cuántos cubos hay ahora?” // “How are you planning to find out how many cubes there are now?”
• “¿Cómo podrías usar lo que hemos aprendido sobre decenas para resolver este problema?” // “How could you use what we've learned about tens to solve this problem?”

Activity Synthesis

• Invite previously identified students to share their representations.
• “¿En qué se parecen estas representaciones? ¿En qué son diferentes?” // “How are these representations the same? How are they different?”

Activity 2: Suma y resta decenas (15 minutes)

Narrative

The purpose of this activity is for students to practice adding and subtracting multiples of 10 from multiples of 10. Students may use any method that makes sense to them, such as using connecting cubes in towers of 10, making base-ten drawings, or adding or subtracting the single-digit number of tens. When students notice they can add tens to tens just like they add single-digit numbers, they look for and make use of the base-ten structure of numbers (MP7).

Action and Expression: Develop Expression and Communication. Identify connections between the different methods students used and the way in which students showed adding or subtracting tens.
Supports accessibility for: Conceptual Processing, Organization

Launch

• Groups of 2
• “Ahora van a practicar sumar y restar con decenas” // “Now you will practice adding and subtracting with tens.”

Activity

• 5 minutes: independent work time
• 5 minutes: partner work time

Student Facing

1. Suma 20:

Ecuación: ____________________________

2. Resta 20:

Ecuación: ____________________________

3. 4 decenas y 5 decenas es ___________

Ecuación: ____________________________

4. 8 decenas quitando 3 decenas es __________

Ecuación: ____________________________

5. $$20 + 60 = \boxed{\phantom{\frac{aaai}{aaai}}}$$
6. $$70 - 20 = \boxed{\phantom{\frac{aaai}{aaai}}}$$
7. $$90 - 70 = \boxed{\phantom{\frac{aaai}{aaai}}}$$
8. $$40 + 40 = \boxed{\phantom{\frac{aaai}{aaai}}}$$

Student Response

If students count on by one or do not find the numbers that make each equation true, but are able to complete the base-ten drawings and addition and subtraction statements, consider asking:

• “¿Como encontraste el número que hace que esta ecuación sea verdadera? ¿Cómo puedes demostrar que la ecuación es verdadera?” // “How did you find the number to make this equation true? How can you prove that the equation is true?”
• “¿Cómo podrías pensar en esta ecuación como sumar (o restar) decenas?” // “How could you think about this equation as adding (or subtracting) tens?”

Activity Synthesis

• Share responses and different methods used for each problem.

Lesson Synthesis

Lesson Synthesis

Display $$4 + 3$$ and $$40 + 30$$.
“Hoy practicamos sumar y restar decenas. ¿En qué se parecen estas expresiones? ¿En qué son diferentes?” // “Today we practiced adding and subtracting tens. How are these expressions the same? How are they different?” (They are both adding 4 + 3. The first one is adding ones and the second is adding tens. You can use 4 tens + 3 tens for the second expression.)

Student Section Summary

Student Facing

Aprendimos cómo mostrar decenas de diferentes formas.

6 decenas

60

Aprendimos cómo sumarle y restarle decenas a otras decenas.

4 decenas y 1 decena son 5 decenas

$$40+10=50$$

5 decenas quitando 2 decenas son 3 decenas

$$50-20=30$$