# Lesson 3

Add or Subtract to Solve Story Problems

## Warm-up: How Many Do You See: Base-ten Diagram (10 minutes)

### Narrative

The purpose of this How Many Do You See is to build on what students know about place value to make sense of a visual representation of two-digit numbers. When students describe how many they see by grouping tens with tens and ones with ones or composing a ten, they show how they look for and make use of base-ten structure (MP7). This will be helpful when students use base-ten representations to compose and decompose a ten in future lessons.

### Launch

• Groups of 2
• “How many do you see? How do you see them?”
• Flash the image.
• 30 seconds: quiet think time

### Activity

• Display the image.
• 1 minute: partner discussion
• Record responses.
• Repeat for each image.

### Student Facing

How many do you see? How do you see them?

### Activity Synthesis

• “How did you know how many were in the third image?”
• “How could we represent this number another way?” (Instead of 10 ones, we could use a ten.)

## Activity 1: Fun at the Zoo (20 minutes)

### Narrative

The purpose of this activity is for students to interpret and solve a story problem by adding or subtracting within 100. Students solve an Add To, Start Unknown problem, one of the more difficult problem types from grade 1. Students begin the activity by looking at the problem displayed, rather than in their books. At the end of the launch, students open their books and work on the problem.

Monitor for students who use methods that show adding or subtracting by place to share in the synthesis. Students who choose to use connecting cubes or base-ten blocks or who draw a diagram to represent the situation are using tools strategically (MP5). During the synthesis, invite all students to explain why these methods work using their understanding of place value.

This activity uses MLR6 Three Reads. Advances: reading, listening, representing. Some students may benefit from continued use of MLR6 to support reading comprehension in Activity 2.

### Required Materials

Materials to Gather

### Launch

• Groups of 2
• “Have you ever been on a field trip? Where did you go?”
• “Did everyone on your field trip stay together the whole time or did you split into smaller groups?”

• Display only the story problem, without revealing the question.
• “We are going to read this problem 3 times.”
• 1st Read: “Some students were on the bus to go to the zoo. Then 34 more students got on. Now there are 55 students on the bus.”
• “What is this story about?”
• 1 minute: partner discussion.
• Listen for and clarify any questions about the context.
• 2nd Read: “Some students were on the bus to go to the zoo. Then 34 more students got on. Now there are 55 students on the bus.”
• “What are all the things we can count in this story?” (number of students who started the story on the bus, number of students who got on next, the total number of students on the bus, the number of buses)
• 30 seconds: quiet think time
• 2 minutes: partner discussion
• Share and record all quantities.
• Reveal the question.
• “What are different ways we can solve this problem?” (We could subtract the number of students who got on the bus second from the total. We could add to the number of students who got on the bus until we get to the total.)
• 30 seconds: quiet think time
• 1–2 minutes: partner discussion

### Activity

• 5 minutes: independent work time
• Monitor for students who:
• use base-ten blocks or base-ten diagrams to show adding tens to tens or ones to ones
• use base-ten blocks or base-ten diagrams to show subtracting from tens or ones from ones

### Student Facing

Some students were waiting on the bus to go to the zoo. Then 34 more students got on. Now there are 55 students on the bus. How many students were on the bus at first?

### Activity Synthesis

• Invite previously identified students to share.
• Record student methods using equations.
• “How are these methods the same?” (They both show tens and ones. They both thought of keeping the tens with the tens and ones with the ones.)

## Activity 2: More Fun at the Zoo (15 minutes)

### Narrative

The purpose of this activity is to solve different story problems by adding or subtracting within 100 without composing or decomposing a ten. Each problem elicits the relationship between addition and subtraction and can be solved with either operation. Students are encouraged to describe methods based on place value and should have access to base-ten blocks. In the synthesis, students compare representations and make connections between concrete representations and drawings.

Invite students to use the Three Reads routine to support reading comprehension. Some students may also benefit from reading the story problems with their partner before working independently.

Representation: Develop Language and Symbols. Synthesis: Invite students to explain their thinking orally, using the connecting cubes or base-ten blocks, while students work independently. Check in frequently to make sure the students are making connections between the story and the cubes or blocks.
Supports accessibility for: Language, Visual-Spatial Processing

### Required Materials

Materials to Gather

• Groups of 2

### Activity

• 6 minutes: independent work time
• 4 minutes: partner discussion
• Monitor for students who use base-ten blocks and students who use base-ten diagrams to represent methods based on adding or subtracting by place.

### Student Facing

Solve each story problem. Show your thinking.

1. There were 65 students in the monkey house. 23 left to see the hippos. How many are still in the monkey house?
2. 58 students went to see the bears. 27 students went to see the lions. How many more students went to see the bears than the lions?
3. Some birds were in cages outside of the bird house. 34 birds were inside the bird house. In all, there were 88 birds. How many were in the cages outside?

### Student Response

If students do not show evidence of grouping tens and ones, consider asking:
• “What did you do to answer the story problem?”
• “How could you use what you know about tens and ones to add or subtract?”

### Activity Synthesis

• Invite previously identified students to share. Consider selecting a student who used base-ten blocks first, followed by a student who used a base-ten drawing.
• If no students use a base-ten drawing, draw one to record the way a student uses their blocks to represent and solve the problem.
• “How are these representations the same?” (Both started by representing 88 birds. _____ moved away 3 tens and 4 ones and _____ crossed out 3 tens and 4 ones.)
• 1 minute: quiet think time
• Share responses.

## Lesson Synthesis

### Lesson Synthesis

Display a base-ten diagram from Activity 2 that was shared in the activity synthesis.

“Let’s come up with equations we could write to match how _____ used their diagram.” ($$80 - 30 = 50$$, $$8 - 4 =4$$, $$50 + 4 = 54$$)

## Student Section Summary

### Student Facing

In this section, we used addition and subtraction to compare and to find unknown values. We used different tools to show how to add and subtract two-digit numbers.

$$50 - 30 = 20$$
$$5 - 4 = 1$$
$$20 + 1 = 21$$