# Lesson 3

## Warm-up: Number Talk: Use Known Sums (10 minutes)

### Narrative

The purpose of this Number Talk is to encourage students to think about using known sums to create easier calculations and to rely on the properties of operations or the relationship between addition and subtraction to mentally solve problems. The methods elicited here will be helpful as students add and subtract within 20.

### Launch

• Display one expression.
• “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

Find the value of each expression mentally.

• $$6 + 7$$
• $$13 - 7$$
• $$7 + 8$$
• $$15 - 7$$

### Activity Synthesis

• “Some of us used $$7 + 8$$ to help with $$15 -7$$. What other subtraction expression does $$7 + 8$$ help with?” ($$15 -8$$. It’s like the other subtraction equation, you just have to find a different unknown addend.)

## Activity 1: Many Ways to Add and Subtract (20 minutes)

### Narrative

The purpose of this activity is for students to add and subtract within 20. Students are encouraged to find sums and differences in as many different ways as they can. They record their thinking and then do a gallery walk to look at their classmates’ work. Set-up for the gallery walk by putting all the posters for a specific expression together. The synthesis focuses on how decomposing a number to lead to a ten can be a helpful method when subtracting within 20 (MP7).
This activity uses MLR7 Compare and Connect. Advances: representing, conversing.

### Required Materials

Materials to Gather

### Launch

• Groups of 2
• Give each group tools for creating a visual display and access to connecting cubes in towers of 10 and singles.

### Activity

MLR7 Compare and Connect
• “Create a visual display that shows your thinking about each expression. You may want to include details such as drawings, numbers, words, or equations to help others understand your thinking.”
• 7 minutes: partner work time
• “Now we will do a gallery walk so you can look at your classmates' work. Look for ways that their work is the same and different.”
• 7 minutes: gallery walk

### Student Facing

Circle 1 addition expression and 1 subtraction expression.

$$5+9$$

$$14-8$$

$$4 + 7$$

$$15-9$$

$$6+ 4 +4$$

$$13-6$$

Find the value of the expressions in as many different ways as you can.

Show your thinking using drawings, number, or words.

### Activity Synthesis

• “What is the same and what is different about how you and your classmates added or subtracted?” (Some of us wrote equations. Some of us made 10 for the addition and subtraction expressions.)
• Display $$13 - 6 = \boxed{\phantom{7}}$$, $$13 - 3 - 3$$, $$10 - 3 = 7$$.
• “How did this student find the difference between 13 and 6?” (They broke 6 into 3 and 3. They took 3 from 13 to get to 10 and then took the other 3 away.)
• “Why is it helpful to look for ways to get to a 10 when subtracting?” (Once you are at 10 it is easy to subtract another number because we know our ten facts.)

## Activity 2: Heads Up (20 minutes)

### Narrative

The purpose of this activity is for students to practice adding and subtracting within 20. Students add to find the sum of two numbers, and either add or subtract to find the unknown addend when one addend and the sum are given.

MLR8 Discussion Supports. Synthesis: Invite students to take turns sharing which equation they would choose and why. Ask students to restate what they heard using precise mathematical language and their own words. Display the sentence frame: “I heard you say . . . .” Original speakers can agree or clarify for their partner.
Engagement: Develop Effort and Persistence: Differentiate the degree of difficulty or complexity. Begin by giving students a subset of the number cards with more accessible values and then introduce the remaining cards once students have written equations for the first subset of cards.
Supports accessibility for: Attention, Conceptual Processing

### Required Materials

Materials to Gather

### Launch

• Groups of 3
• Give students number cards and access to connecting cubes in towers of 10 and singles.
• “We are going to play a game called Heads Up. This game is played with three students.”
• Demonstrate with two students. Ask each student to choose a card without looking at it and hold it up to their foreheads.
• “My partners have each chosen a card. My job is to find the sum and tell my group. Then, each of my partners use the other player’s number and the sum to determine what number is on their head. Then we all write the equation that represents what we did.”
• Demonstrate writing an equation to show how you found the sum of the two numbers.
• Ask students how they would find the number on each player's head and record the equations.
• “After each round switch roles and play again.”

### Activity

• 15 minutes: small-group work time

### Student Facing

Write an equation for each round you play.

Round 1: ________________________________

Round 2: ________________________________

Round 3: ________________________________

Round 4: ________________________________

Round 5: ________________________________

Round 6: ________________________________

### Student Response

If students only write equations using one operation when it is their turn to find an unknown addend, (for example, they only write subtraction equations), consider asking:

• “What is another equation you could use to figure out what number is on your head?”
• “What is a way you could use subtraction (or addition) to find the number that is on your head?”
• “Which way is easier for you to think about figuring out the number on your head? Why?”

### Activity Synthesis

• “During one round of Heads Up, Diego’s partner had a 3 on their card. Diego was told that the sum of their numbers was 12. What equations can Diego use to figure out what number is on his card?” ($$12 - 3 = \underline{\hspace{1 cm}}$$ or $$3 + \underline{\hspace{1 cm}} = 12$$)
• “Explain which of these equations you would use to find the unknown number.” (I would use $$12 - 3$$ because it is really easy to count back 3. I would choose $$3 + \underline{\hspace{1 cm}} = 12$$ because I prefer adding. I would add 7 to get to 10 and then 2 more.)

## Lesson Synthesis

### Lesson Synthesis

“How does knowing addition and subtraction facts within 10 help you add and subtract within 20?” (I know all the different ways to make ten, so I can look for ways to get to a ten when I add or subtract. It helps me think of ways to make easier sums or differences. I can decompose one number and add or subtract in parts.)