Lesson 14
Demos sentido a la resta de números decimales
Warm-up: Verdadero o falso: Diferencias decimales (10 minutes)
Narrative
The purpose of this True or False is for students to apply their understanding of place value to subtraction equations with decimals. The names of the decimals will likely suggest a strategy using whole number subtraction. For example \(0.61 - 0.02 = 0.59\) can be read as “61 hundredths minus 2 hundredths equals 59 hundredths,” which students will recognize is true. Alternatively they can think about place value. For example, 0.5 is 5 tenths or 50 hundredths and 50 hundredths minus 1 hundredth is 49 hundredths.
Launch
- Display one statement.
- “Hagan una señal cuando sepan si la afirmación es verdadera o no, y puedan explicar cómo lo saben” // “Give me a signal when you know whether the statement is true and can explain how you know.”
- 1 minute: quiet think time
Activity
- Share and record answers and strategy.
- Repeat with each statement.
Student Facing
En cada caso, decide si la afirmación es verdadera o falsa. Prepárate para explicar tu razonamiento.
- \(0.5 - 0.01 = 0.4\)
- \(0.61 - 0.02 = 0.59\)
- \(1 - 0.07 = 0.93\)
Student Response
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Activity Synthesis
- Display first equation.
- “¿Cómo usaron lo que saben sobre el valor posicional para decidir si la ecuación es verdadera?” // “How did you use what you know about place value to decide if the equation is true?” (I know 5 tenths is 50 hundredths and if I subtract 1 hundredth that’s 49 hundredths so it’s false.)
Activity 1: La diferencia (20 minutes)
Narrative
The purpose of this activity is for students to subtract decimals in a way that makes sense to them. Students should be encouraged to use whatever strategies make sense to them, including using place value understanding and the relationship between addition and subtraction. Strategies students may use include
- using hundredths grids (MP5)
- using place value and writing equations
This activity uses MLR7 Compare and Connect. Advances: Representing, Conversing.
Supports accessibility for: Conceptual Processing, Language, Attention
Required Materials
Launch
- Groups of 2
- Give each group of students a piece of chart paper, colored pencils, crayons or markers, and access to grids.
- 5 minutes: independent work time
Activity
- “Creen una presentación visual que muestre cómo pensaron. Incluyan detalles, como notas, diagramas y dibujos, para ayudar a los demás a entender sus ideas” // “Create a visual display that shows your thinking. You may want to include details such as notes, diagrams, and drawings to help others understand your thinking.”
- 2–5 minutes: partner work time
- 5–7 minutes: gallery walk
- Monitor for students who:
- use strategies like the ones listed in student solutions
Student Facing
- Encuentra el valor de \(2.26 - 1.32\). Explica o muestra cómo razonaste.
- ¿Qué preguntas tienes sobre la resta de números decimales?
Student Response
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Advancing Student Thinking
If students do not have a strategy to get started, ask, “¿La diferencia es mayor que 1 o menor que 1? ¿Cómo lo sabes?” // “Is the difference greater than or less than 1? How do you know?”
Activity Synthesis
- Refer to visual displays that students created.
- “¿En qué se parecen y en qué son diferentes las estrategias que se usaron para encontrar el valor de la diferencia?” // “What is the same and what is different about the strategies used to find the value of the difference?” (They all pay close attention to place value and the meaning of each digit in the numbers. Some of the displays use subtraction while some of them use addition. They add and subtract different pieces sometimes in different order. Some of the displays have diagrams and some don’t.)
- Display image from students solution.
- “¿Cómo se ve la diferencia \(2.26 - 1.32\) en el diagrama?” // “How does the diagram show the difference \(2.26 - 1.32\)?” (It shows 2.26 and then it takes away 1.32. What’s left is the difference.)
- “¿Qué diferencia hay entre usar el diagrama y usar ecuaciones?” // “How is the diagram different from using equations?” (The 1 and 32 hundredths can be crossed out anywhere that’s convenient and then you just count what’s left. You don't have to think as strategically about what to take away or in what order.)
Activity 2: Números objetivo: Resta décimas o centésimas (15 minutes)
Narrative
The purpose of this activity is for students to practice subtracting decimals to the hundredth. Students play a new stage of the center Target Numbers. Students played a previous stage that involved adding tenths or hundredths. The game begins when students roll a number cube and decide whether they want the number to represent tenths or hundredths. Then, they subtract the number from 2. They roll the number cube six times and try to make a final difference that is as close to 1 as possible, without being less than 1. Students make strategic choices about which value to assign the number they rolled and adapt their strategy throughout the game. For example, here is a sample record of a game.
number rolled |
0.1 | 0.01 | equation to represent the difference |
---|---|---|---|
4 | 0.4 | \(2-0.4=1.6\) | |
1 | 0.1 | \(1.6-0.1=1.5\) | |
3 | 0.3 | \(1.5-0.3=1.2\) | |
5 | 0.05 | \(1.2-0.05=1.15\) | |
2 | 0.02 | \(1.15-0.02=1.13\) | |
5 | 0.05 | \(1.13-0.05=1.08\) |
Some students may notice that the strategy of this game is identical to the addition game they played in an earlier lesson, the only difference being that here they start at 2 and subtract whereas with the addition game they started at 0 and add (MP8).
Required Materials
Launch
- Groups of 2
- Give each group 1 number cube.
- “Vamos a jugar una nueva etapa del juego ‘Números objetivo’. Leamos las instrucciones y juguemos una ronda juntos” // “We’re going to play a new stage of the game called Target Numbers. Let’s read through the directions and play one round together.”
- Read through the directions with the class and play a round with the class:
- Display each roll of the number cube.
- Think through your choices aloud.
- Record your move and score for all to see.
- “Ahora jueguen el juego con su compañero” // “Now, play the game with your partner.”
Activity
- 8–10 minutes: partner work time
- Monitor for students who:
- go under 1 before their sixth roll and describe how they could have changed the placement of a number from the tenths place to the hundredths place to possibly impact the outcome of the game
- have a final difference that is not close to 1 and describe how they could have strategically changed the placement of a number from the hundredths place to tenths place to possibly impact the outcome of the game
Student Facing
Instrucciones:
- Jueguen una ronda de “Números objetivo”.
- Compañero A
- Empieza en el 2. Lanza el dado numérico. Escoge si restarle ese número de décimas o de centésimas al número inicial.
- Escribe una ecuación que represente la diferencia.
- Jueguen por turnos hasta que hayan jugado 6 rondas.
- En cada ronda, la diferencia de la ecuación anterior es el número con el que se empieza en la nueva ecuación.
- Gana el que tenga la diferencia más cercana a 1, sin que esta diferencia sea menor que 1.
- Compañero A
- Describe una jugada que podrías haber hecho de otra forma para cambiar el resultado del juego.
Student Response
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Advancing Student Thinking
If students are not being strategic about their placement of the digits, ask, “¿Por qué le diste ese valor al número que obtuviste?” // “Why did you choose that value for the number you rolled?”
Activity Synthesis
- Ask previously selected students to share their reflections on the game.
- Recall the addition game students played three lessons earlier, choosing tenths or hundredths with a target number of 1.
- “¿En qué se parecen el juego de la resta y el juego de la suma?” // “How is the subtraction game the same as the addition game?” (I have 6 rolls of the dice and I get to choose tenths or hundredths each time. The strategy is the same because in both cases I want the 6 decimals to add up to close to 1 without going over.)
- “¿En qué son diferentes el juego de la resta y el juego de la suma?” // “How is the subtraction game different than the addition game?” (Instead of adding the decimals starting with 0, I am subtracting them starting with 2. Sometimes I have to break up a 1 or a tenth. In the addition game I sometimes made a 1 or a tenth.)
- “¿Cuál juego les gustó más?” // “Which game did you prefer?” (I liked the addition game because it was easier to add the decimals than to subtract.)
Lesson Synthesis
Lesson Synthesis
Create a large chart titled “Resta de números decimales” // “Decimal Subtraction”.
“¿En qué se parecen la resta de números decimales y la resta de números enteros?” // “How is subtracting decimals the same as subtracting whole numbers?” (You have to pay attention to the place values and make sure you are subtracting digits with the same place value. Sometimes you have to decompose a unit.)
“¿En qué son diferentes?” // “How is it different?” (There are more places to pay attention to and I have to remember how many tenths are in a whole, how many hundredths are in a tenth.)
“¿En qué se parecen la resta de números decimales y la suma de números decimales? ¿En qué son diferentes?” // "How is subtracting decimals the same as adding decimals? How is it different?” (I have to think about place value in the same way. The only difference is that I’m subtracting now instead of adding. So I have to decompose sometimes while with addition I compose sometimes.)
Record responses on poster.
Cool-down: Resta (5 minutes)
Cool-Down
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