Lesson 1

Sumas y diferencias hasta 20

Warm-up: Conversación numérica: Restemos hasta 20 (10 minutes)


This Number Talk encourages students to think about how they may use known sums and differences to find the value of other sums and differences. This understanding will be helpful as students continue building fluency with addition and subtraction within 20.


  • 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


  • Record answers and strategies.
  • Keep expressions and work displayed.
  • Repeat with each expression.

Student Facing

Encuentra mentalmente el valor de cada expresión.

  • \(10 - 5\)
  • \(11 - 5\)
  • \(12 - 6\)
  • \(13 - 6\)

Student Response

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Activity Synthesis

  • “¿Cómo les ayudó la tercera expresión a encontrar el valor de la última expresión?” // “How did the third expression help you find the value of the last expression?”

Activity 1: Compruébalo tú mismo (20 minutes)


The purpose of this activity is for students to identify the addition facts within 20 that they do not yet know from memory. They write these sums on index cards which can be used to help students build fluency throughout the section. Students should store these cards to use again in an upcoming lesson. The number choices in this activity include some of the facts that students may still be working to recall from memory at this point in the school year. If desired, the inventory of sums that students complete at the beginning of the activity could be replaced with a list of all sums within 20 or a smaller set of sums that best fit the needs of your students.


  • Groups of 2
  • Give students access to index cards.


  • “Encuentren mentalmente el valor de cada suma” // “Find the value of each sum mentally.”
  • 2 minutes: independent work time
  • “Marquen cualquier suma que no hayan podido recordar inmediatamente” // “Circle any sums that you did not remember right away.”
  • Share answers. Prompt students to also circle any sums that they did not find correctly.
  • “Escriban en tarjetas distintas las expresiones que hayan marcado. En un lado escriban la expresión y en el otro el valor de la expresión. Luego, pueden usar las tarjetas para practicar estas sumas” // “Write each expression that you circled on one side of an index card. Write the value of the expression on the other side. Then you can use the cards to practice these sums.”
  • “Si no marcaron ninguna expresión, hagan tarjetas para algunas de las expresiones más complicadas de recordar. Hagan tarjetas para esas sumas y piensen en los métodos que usan para recordarlas rápidamente. Prepárense para compartir sus ideas” // “If you did not circle any expressions, make cards for some of the expressions that are the trickiest to remember. Make cards for these sums and think about what methods you use to remember these sums quickly. Be prepared to share.”
  • 6 minutes: independent work time
  • Monitor for the sums that several students identify as a sum they need to work on.
  • “Compartan con su compañero lo que pensaron” // “Share your thinking with your partner.”
  • 2 minutes: partner discussion

Student Facing

  1. Encuentra mentalmente el valor de cada suma.

    \(6 + 6\)

    \(9 + 9\)

    \(7 + 7\)

    \(8 + 8\)

    \(5 + 5\)

    \(9 + 8\)

    \(7 + 8\)

    \(8 + 5\)

    \(6 + 5\)

    \(7 + 6\)

    \(7 + 9\)

    \(6 + 8\)

    \(9 + 2\)

    \(7 + 5\)

    \(8 + 4\)

    \(2 + 9\)

    \(9 + 7\)

    \(6 + 9\)

    \(8 + 3\)

    \(9 + 4\)

    \(8 + 6\)

    \(3 + 9\)

    \(5 + 6\)

    \(9 + 5\)

    \(5 + 9\)

    \(9 + 6\)

    \(8 + 9\)

    \(6 + 7\)

    \(5 + 7\)

    \(8 + 7\)

  2. Marca todas las sumas que no hayas podido recordar inmediatamente.

    Escribe cada una de las sumas que hayas marcado en una tarjeta.

    Usa las tarjetas para mejorar tu fluidez con estas sumas.

Student Response

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Activity Synthesis

  • Display a list of sums that many students are still working on.
  • “Muchos estudiantes escribieron estas sumas en sus tarjetas. Escojan una de las expresiones y prepárense para explicar cómo pueden recordar la suma” // “A lot of students wrote these sums on their cards. Choose one of the expressions and be ready to explain how you can remember the sum.”
  • Share and record responses.

Activity 2: Giremos la ruleta y encontremos el número desconocido (15 minutes)


The purpose of this activity is for students to practice finding unknown addends within 20. Students use spinners to determine the total and one of the addends. They write addition equations or subtraction equations to represent how they found the unknown addend. Students connect the different equations in the synthesis and compare different methods for finding the unknown addend mentally.

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

Action and Expression: Develop Expression and Communication. Give students access to a number line or a number bond mat with base ten blocks. Allow students to recreate the problems in a concrete manner and continue to build a deeper understanding if needed.
Supports accessibility for: Conceptual Processing, Memory, Organization

Required Materials

Materials to Gather

Materials to Copy

  • Spin and Find the Missing Number Spinners


  • Groups of 2
  • Give each group the spinners and a paper clip.
  • “Juguemos un juego para practicar nuestros hechos de suma y de resta” // “Let’s play a game to practice our addition and subtraction facts.”
  • “Giren la primera ruleta para obtener su total. Giren la segunda ruleta para obtener un sumando” // “Spin the first spinner to get your total. Spin the second spinner to get one addend.”
  • Demonstrate spinning both spinners and naming your total and first addend.
  • “Ambos jugadores encuentran mentalmente el sumando desconocido” // “Both players mentally find the missing addend.”
  • Think aloud, finding the missing addend.
  • “Compartan con su compañero cómo pensaron. Si ambos están de acuerdo en cuál es el sumando desconocido, escriban y compartan ecuaciones que muestren cómo encontraron el número desconocido” // “Share your thinking with your partner. If you both agree on the missing addend, write and share equations that show how you found the missing number.”

MLR8 Discussion Supports

  • Display sentence frames to support small-group discussion:
    • “Estoy de acuerdo porque . . .” // “I agree because . . . .”
    • “Estoy en desacuerdo porque . . .” // “I disagree because . . . .”
  • Demonstrate writing an equation that best matches the method you used to find the missing addend.


  • 10 minutes: partner work time
  • Monitor for students who:
    • decompose to get to a ten
    • use known addition or subtraction facts
    • make equivalent but easier sums

Student Facing

  • Gira la ruleta de arriba para obtener el total.
  • Gira la ruleta de abajo para obtener un sumando.
  • Encuentra el sumando desconocido.
  • Escribe una ecuación para mostrar cómo encontraste el sumando desconocido.
mis ecuaciones las ecuaciones de mi compañero

Student Response

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Advancing Student Thinking

If students disagree about the unknown number, encourage students to use the sentence frames and create drawings or diagrams to explain why they think their number would make the equation true.

Activity Synthesis

  • “Lean sus ecuaciones. ¿Qué ecuaciones pudieron resolver inmediatamente?” // “Read your equations. Which equations did you know right away?”
  • “¿Qué hicieron cuando no sabían cuál era el número desconocido?” // “What did you do when you did not know the unknown number?”
  • Invite previously identified students to share methods.

Lesson Synthesis

Lesson Synthesis














“¿Cuáles expresiones agruparían? ¿Por qué?” // “Which expressions would you group together? Why?” (A and E because you could use \(9 + 8 = 17\) to find \(17 - 9\). B and F because you could use \(7 + 7\) to find the value of \(14 - 7\). B, D, and E because they are all addition.)

Cool-down: Suma y resta con fluidez (5 minutes)


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