Lesson 22

The purpose of this Number Talk is to elicit strategies and understandings students have for finding the difference of two numbers. These understandings develop fluency and will be helpful later in this lesson when students subtract from teen numbers. 

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

• “¿Cómo se puede usar \(14 - 4 = 10\) para encontrar la diferencia en \(14 - 5\)?” // “How can you use \(14 - 4 = 10\) to find the difference in \(14 - 5\)?” (I know \(14 - 4\) is 10, so if I subtract 1 more, the answer is 9.)
• “¿Cómo se puede usar \(17-7= 10\) para encontrar la diferencia en \(17-9\)?” // “How can you use \(17-7= 10\) to find the difference in \(17-9\)?” (\(17 - 7\) is 10, so I am subtracting 2 more in \(17 - 9\). So I can take 2 from 10 to get the answer.)

The purpose of this activity is for students to solve a Take From, Result Unknown problem, which requires decomposing a ten, in a way that makes sense to them. The problem is presented with an image that encourages students to think about 16 as a ten and 6 ones. They create a poster to share how they solved the problem, and participate in a gallery walk to see how their classmates solved. As students are working, the teacher monitors for methods to analyze during the activity synthesis. As they do so, students connect addition and subtraction and see how either can be used to solve the problem (MP7).

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

• Groups of 2
• Give students access to double 10-frames and connecting cubes or two-color counters.

• Read the task statement
• 5 minutes: partner work time
• Monitor for students who
  • take away 7 from 16
  • count on from 7 to 16
  • take away 6 to get 10 and then subtract 1 more
  • count on from 7 to 10 and then add 6 more

• Give each group tools for creating a visual display.
• “Hagan un póster que muestre cómo pensaron sobre el problema. Asegúrense de hacerlo de una forma que los demás entiendan” // “Create a poster that shows your thinking about the problem. Make sure to show your thinking in a way others will understand.”
• 5 minutes: partner work time
• “Mientras caminan y observan los pósteres, piensen en qué se parecen los pósteres y en qué son diferentes” // “As you walk around and look at the posters, think about how the work is the same and different.”
• 5 minutes: gallery walk
• “¿En qué se parecen y en qué son diferentes las representaciones?” // “What is the same and what is different about the representations?” (They all showed 16 and 7. They used math tools to represent he problem. Some people used addition facts they knew, some counted up, some took away.)

The purpose of this activity is for students to subtract a one-digit number from a teen number. Students choose a teen number card, then choose a number card to subtract from the teen number. They write an equation to represent each round. The equations students write may involve subtraction, or addition with a missing addend. During the synthesis, students share their methods. When explaining, students have opportunities to revise their language to make their explanations more precise and clear (MP6).

Students play this game again in the next lesson with a double 10-frame, so keep the materials organized for future use.

• Each group of 2 needs a set of Number Cards 0–10 and a set of Number Cards 11–20 used in a previous lesson.

• Groups of 2
• Give each group a set of number cards 0–10, a set of number cards 11–20, and access to double 10-frames and connecting cubes or two-color counters.
• “Vamos a jugar un juego para practicar la resta. Para jugar, toman una tarjeta de números del 11 al 19. Luego, toman una tarjeta de números del 0 al 9 y le restan este número al número del 11 al 19. Después, escriben una ecuación para mostrar la diferencia” // “We’re going to play a game to practice subtracting. To play this game you pick a teen number card. Then you pick a number card 0–9 and subtract that number from your teen number. Write an equation to show the difference.”
• “Al final del juego, van a elegir un turno y a mostrar cómo encontraron la diferencia” // “At the end of the game, you are going to pick one turn and show how you found the difference.”

• 10 minutes: partner work time
• Monitor for students who:
  • take away 
  • count on 
  • take away to get to 10, then take away some more
• “Individualmente, escojan su ronda favorita. Muestren cómo encontraron el valor de la diferencia. Usen dibujos, números o palabras” // “On your own, pick your favorite round. Show how you found the value of the difference using drawings, numbers, or words.”
• 3 minutes: independent work time

• Invite previously identified students to share.
• “¿Cuál de los métodos que vieron hoy les gustaría probar?” // “What method did you see today that you would like to try?”