# Lesson 3

Prime and Composite Numbers

## Warm-up: Choral Count: Twos and Fives (10 minutes)

### Narrative

The purpose of this Choral Count is to invite students to practice counting by 2 and 5 and notice patterns in the count. These understandings help students develop fluency and will be helpful later when students find factor pairs.

When students predict common multiples for 2 and 5 based on the numbers recorded from the count and what they know about multiplication, they look for and express regularity in repeated reasoning (MP8).

### Launch

• “Count by 2, starting at 0.”
• Record as students count.
• Stop counting and recording at 30.
• “Count by 5, starting at 0.”
• Record as students count.
• Stop counting and recording at 75.

### Activity

• “What patterns do you see in the individual counts?”
• 1–2 minutes: quiet think time
• Record responses.
• “What patterns do you see between the two counts?”
• 1–2 minutes: quiet think time
• Record responses.

### Activity Synthesis

• If it doesn’t come up in the student responses, ask: “How many twos did it take to get to 10? How many fives did it take to get to 10?” (It took 5 twos and 2 fives to get to 10.)
• “Ten is a multiple of 2 and 5. Do you notice any other multiples of both 2 and 5?” (20 and 30 are on both lists.)
• 1 minute: partner discussion
• Record responses.
• “If the counts continue, what other numbers would you see that are multiples of both 2 and 5?” (I think 40 would be the next common multiple because the multiples are going up by 10. I think 100 would be a common multiple because $$2\times50 = 100$$ and $$5 \times 20 = 100$$.)
• 2 minutes: partner discussion
• Record responses.

## Activity 1: Card Sort: Area (15 minutes)

### Narrative

The purpose of this activity is for students to learn about prime numbers and composite numbers. Students are given a set of cards with rectangles on them. They sort the rectangles by area and then attempt to draw an additional rectangle for each category. They notice that some areas can be represented by more than one rectangle and some areas can only be represented by one rectangle.

During the synthesis, highlight that the side lengths of each rectangle represent one factor pair (each pair of side lengths should be used only once), and that the area of each rectangle represents a multiple of each side length. Students learn that a number with only one factor pair—1 and the number itself—is a prime number, and a number with more than one factor pair is a composite number.

Here is an image of the cards for reference.

MLR8 Discussion Supports. Invite students to take turns selecting a rectangle, and explaining how they should sort it to their partner. Display the following sentence frames: “This rectangle belongs with _____ , because . . .” Encourage students to challenge each other when they disagree.
Engagement: Develop Effort and Persistence. Chunk this task into manageable parts to support organizational skills in problem solving. Some students may benefit from explicit guidance for how to begin. For example, before sorting, students can find the area of each rectangle.
Supports accessibility for: Conceptual Processing, Visual-Spatial Processing, Organization

### Required Materials

Materials to Gather

Materials to Copy

• Card Sort: Area

### Required Preparation

• Create a set of cards from the blackline master for each group of 2.

### Launch

• Groups of 2
• Give each group a set of cards from the blackline master.
• “Sort the cards into categories in any way that makes sense to you.”
• 2 minutes: partner work time
• Ask students to share ways in which they sorted.

### Activity

• “If you did not already, sort the rectangles by their area.”
• 3–5 minutes: partner work time
• Ask students to check their work with another group to make sure the cards in each category match.
• “Now, create at least one rectangle to add to each category in your card sort.”
• 3–5 minutes: partner work time
• Observe the rectangles students add to each category. Monitor for students who notice that no new rectangles could be drawn for the area of 7 square units.

### Student Facing

Your teacher will give you a set of cards to sort.

1. Sort the cards by area. Record your sorting results. Be prepared to explain your choices.
2. For each group of sorted cards, think of at least one more rectangle. Name its length and width. Be prepared to explain your reasoning.

### Activity Synthesis

• Select 2–3 students to share the rectangles they added to each category.
• “Why were you able to create more rectangles for some areas and not others?” (Some of the numbers had more factor pairs. For some numbers, there was only one possible factor pair.)
• Revoice student reasoning. “Only one rectangle can be made for the area of 7. Numbers like this are called prime numbers. Prime numbers have only one factor pair: 1 and itself.”
• “Numbers like 15 that have more than one factor pair are called composite numbers.”
• “What other composite numbers did you work with? How do you know they are composite?” (Twenty-four is a composite number because I can make 2 rows of 12 or 4 rows of 6. Eighteen is composite because it has factor pairs of 2 and 9 and 3 and 6.)

## Activity 2: Prime or Composite? (20 minutes)

### Narrative

In this activity, students use area of rectangles to find all of the factor pairs of a given whole number and decide if the number is prime or composite. The synthesis focuses on finding all possible rectangles for a given area as a strategy to find all the factor pairs of a number. Students may notice that they do not need to find all possible rectangles to determine whether a number is prime or composite.

### Required Materials

Materials to Gather

### Launch

• Groups of 2
• “If you were given a number that is the area of rectangle, how could you find out how many rectangles with that area can be made?” (Test it out with tiles. Think about factor pairs for the number.)
• 1 minute: partner discussion
• Share and record responses.

### Activity

• “Work with your partner to complete this table. Inch tiles and grid paper are available if you’d like them.”
• 10 minutes: partner work time
• Monitor for different ways students find the number of rectangles, such as:
• building the rectangles from inch tiles
• drawing rectangles on grid paper
• drawing rectangles freehand
• listing the factor pairs of the number and knowing that one rectangle corresponds to each pair

### Student Facing

The table shows different areas. How many rectangles can be made for each area?

Complete the table and be prepared to explain or show your reasoning.

Rectangles with the same pair of side lengths should be counted only once. For example, if you count a rectangle with 4 units across and 6 units down, you don’t need to also count a rectangle with 6 units across and 4 units down.

area how many rectangles? prime or composite?
2 square units
10 square units
48 square units
11 square units
21 square units
23 square units
60 square units
32 square units
42 square units
31 square units
56 square units

### Activity Synthesis

• Invite 3–4 groups share their strategy for finding the number of rectangles for a given area.
• “How does the number of factor pairs relate to the number of rectangles?” (The side lengths of each rectangle is a factor pair. So finding all the rectangles would give us all the factor pairs. Or, finding all the factor pairs of the number would tell us how many rectangles have that number for their area.)
• “What are all of the prime numbers in our list? How do we know they are prime?” (2, 23, 31. They each only have one set of side lengths, 1 and the number itself.)
• “What do you notice about the prime numbers?” (They are odd numbers except the number 2.)
• “What is the smallest prime number in our set? Is it the smallest prime number?” (2. I don’t know. Is 1 a prime number?)
• Display a rectangle with an area of 1 square unit.
• “What are the side lengths of a rectangle with an area of 1 square unit?” (1 and 1)
• “Since 1 only has 1 factor, it doesn’t have any factor pairs, so it is neither prime nor composite.”
• “What are all the composite numbers in our set? How do we know they are not prime?” (10, 48, 21, 60, 32, 42, 56. They each have more than 1 factor pair.)

## Lesson Synthesis

### Lesson Synthesis

“Today we learned about prime and composite numbers.”

“How does finding all the rectangles with a certain area tell us if the value of the area is prime or composite?” (The side lengths of each rectangle are a factor pair of the area. If we can find more than one rectangle with that area, that means the number has more than one factor pair and is composite. If we can find only one rectangle, the number is prime.)

“What questions do you still have about these types of numbers?”

## Student Section Summary

### Student Facing

In this section, we used our understanding of the area of rectangles to learn about factors, multiples, factor pairs, prime numbers, and composite numbers.

If we know the side length of a rectangle, we can find the areas that the rectangle could have. For instance, a rectangle with a side length of 3 could have an area of 3, 6, 9, 12, 15, or other numbers that result from multiplying of a whole number and 3. We call these numbers multiples of 3.

If we know the area of a rectangle, we can find the side lengths that it could have. For example, a rectangle with an area of 24 square units can have side lengths of 1 and 24, 2 and 12, 3 and 8, or 4 and 6. We call these possible pairs of side lengths the factor pairs of 24.

We also learned that a number that has only one factor pair—1 and the number itself—is called a prime number. For instance, 5 is prime because its only factor pair is 1 and 5.

A number that has two or more factor pairs is a composite number. For instance, 15 is composite because its factor pairs are 1 and 15, and 3 and 5.