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## Chairs Around The Table

• Lesson
3-5
1

Students investigate the number of chairs that can be placed around an arrangement of square tables. Three related problems in this lesson yield different linear relationships for students to discover.

Using a context of chairs around square tables, students will be exposed to three different linear patterns in this lesson. The patterns vary slightly from situation to situation, and the third situation allows students to determine a solution in multiple ways, in the end leading to an intuitive understanding of perimeter.

Present the following situation to students:

At Pal-a-Table, a new restaurant in town, there are 24 square tables. One chair is placed on each side of a table. How many customers can be seated at this restaurant?

Show an arrangement of one table with four chairs. If your room contains large square tables at which students work in groups, use them as a demonstration. If not, you can draw a picture on the chalkboard, or you can use pattern blocks or other transparent manipulatives on the overhead projector.

When all students understand how chairs are placed, ask, "If there were 24 tables in a room, how many chairs would be needed?" Depending on students’ understanding of multiplication, they may immediately realize that the answer is 24 × 4 = 96. If not, work with the class to complete a table, as follows:

 Tables Chairs 1 4 2 8 3 12 4 16 5 20

From this table, students should realize that the number of chairs is equal to four times the number of tables. Alternatively, they might recognize that each time a table is added, four chairs are added. If there are some students who use each approach, this is a good opportunity to reinforce the connection between multiplication and repeated addition. That is,

 2 × 4 = 4 + 4 3 × 4 = 4 + 4 + 4 4 × 4 = 4 + 4 + 4 + 4 5 × 4 = 4 + 4 + 4 + 4 + 4

and so on.

Ask students to explain their observations. "What is the pattern? How can you find the number of chairs for any number of tables?" [Multiply the number of tables by 4. If there are 24 tables, for instance, the number of chairs is 96. If there are n tables, the number of chairs is 4n.]

After the original problem has been solved ("How many people can be seated at this restaurant?"), explain to students that the restaurant needs some additional help. Pose the following problem:

Pal-a-Table has a problem. For large groups, they must push some of the tables together to make a longer table. As before, they place one chair on each side of the table. How many tables would be needed for a group of 18 people?"

Again, show students an example of the situation. Explain that for just one square table, again four chairs are needed, but when two tables are pushed together, six chairs are needed.

Ask, "How many chairs would be needed if three square tables were pushed together? What about four tables? …five? How would you determine the number of chairs needed for any number of tables?"

At this point, divide students into groups. Allow them to explore various arrangements of tables and chairs using pattern blocks or the Chairs Around a Table Applet. This online activity allows students to check their work and explore the various situations described in this lesson.

As students work, they should keep an organized record of their data. Before they begin to explore, you may wish to discuss how a table or chart could be used to keep their data organized. Allow students to investigate the relationship between tables and chairs, and circulate as they explore. Make sure that every group is working toward finding a general relationship between the number of chairs and the number of tables.

After the exploration, conduct a class discussion to reveal the relationship between chairs and tables. Ask, "If you know the number of tables, how can you determine the number of chairs?" Allow students to suggest relationships, and keep a record of their suggestions on the overhead projector. Then, allow others to agree or disagree with the suggestions. Continue until the class reaches a consensus on which relationships are correct. (If students believe that a suggested relationship is correct, they should be able to show that it is true for the data they collected or provide an explanation of how they know it is true.)

As students suggest answers using words, convert their answers to algebraic expressions using variables. For instance, if a student says, "You find the number of chairs by multiplying the number of tables by 2, and then adding 2," then you might write either of the following on the overhead projector:

chairs = 2 × tables + 2

or

c = (2×t)+2

Although students in grades 3‑5 are not expected to understand symbolic manipulation, these examples will lay the conceptual foundation for understanding the use of variables.

Students may suggest several correct rules for determining the number of chairs. One student may realize that there are two chairs on the sides of each table, as well as an additional chair on each end, leading to the rule c = (2 × t) + 2. This is shown in the first diagram below. Another student may realize that there are three chairs around each end table, and there are two chairs on the sides of each middle table, leading to the rule c = 3 × 2 + 2 × (t -  2). This is shown in the second diagram below. Although students may not be able to verify that these relationships are algebraically equivalent, they should realize that both yield the same solutions.

Once students have discovered acceptable relationships, they should use them to determine the number of chairs when the number of tables is known, and vice versa. To ensure student understanding, ask the following two questions:

• How many chairs would be needed for 24 tables?

[Two chairs are needed on the sides of each table, so that is 2 ×  24 = 48 chairs. An additional two chairs are needed, one on each end, for a total of 48 + 2 = 50 chairs.]

• How many tables would Pal-a-Table have to push together for a group of 18 people?

[When tables are pushed together, one person can sit on each end. That leaves 18 ‑ 2 = 16 people to be seated on the sides of the tables. Two chairs can be placed on either side of each table, and 16 ÷ 2 =  8. Therefore, 8 tables are needed for a group of 18 people.]

Finally, allow students to consider arrangements in which square tables are connected to form the outside border of a rectangle. Pose the following problem to students:

Customers at Pal-a-Table like that tables can be combined for larger groups, but they don’t like that tables are only arranged end-to-end to form a long chain. One patron suggests that tables should instead be arranged in a rectangular pattern with chairs placed around the outside.

Demonstrate some examples on the overhead projector or chalkboard. An arrangement showing 14 tables and 18 chairs is shown below.

Ask, "How many chairs would you need when tables are arranged in rectangular patterns like this?" Again, allow students time to explore various arrangements in their groups using pattern blocks or the Patch Tool Interactive.

As with the previous problem, students may discover various relationships that allow the number of chairs to be determined. If students are familiar with perimeter, then they may realize that the number of chairs is dependent on two variables, namely length and width. The arrangement above can be thought of as a rectangle with length 5 and width 4. The length contributes five chairs on two sides, and the width contributes four chairs on two sides, so the total number of chairs is 2(5) + 2(4) = 18. (Coincidentally, the perimeter of a 5 × 4 rectangle is 18 units.) In general, students should realize that the number of chairs for such an arrangement is equal to twice the sum of the length and width.

Alternatively, students may approach this situation like the previous one and realize that each of the four corner tables has two chairs next to it, but all other tables have only one chair. That gives a total of 4(2) + 10(1) = 18 chairs.

Similarly, students could realize that all tables have one chair next to them, but the four corner tables have an "extra" chair, giving a total of 14(1) + 4(1) = 18 chairs.

If the number of chairs in the length and width of the arrangement are represented by m and n, these three approaches lead to the following symbolic rules, respectively:

c = 2m + 2n
c = 8 + 2(m - 2) + 2(n - 2)
c = t + 4

Note that the last of these symbolic rules indicates that the number of tables (t) is four less than the number of chairs. But the number of tables is given by t = 2m + 2n ‑ 4, so the last expression could be represented by:

c = (2m + 2n ‑ 4) + 4

A whole-class discussion or individual group presentations are important after this exploration. Because there are various ways that students could represent the relationship between chairs and tables, it is important that all students are exposed to other students’ strategies.

• Pattern Blocks (squares and triangles)
• Computers or tablets with internet connection

Assessment Options

1. Require each group of students to present their findings to the class. Each group could create a poster to explain how they determined the relationship between the number of chairs and the number of tables.
2. Have students write a letter to the restaurant explaining what they learned.

Extensions

1. Have students explore the following questions:
• Which arrangement of 24 tables will allow the fewest chairs to be placed around it?
• Which arrangement of 24 chairs will allow the most chairs to be placed around it?
• In general, describe the arrangement that will allow for the fewest and most chairs for a particular number of tables. Is it a rectangle, a square, or some other shape?
2. Allow students to suggest another arrangement of tables, and have them determine a rule for the number of chairs that could be placed around the tables.
3. If the tables are not squares, but are instead triangles, trapezoids, hexagons, or some other shape from the Patch Tool, how many chairs could go around each table? How many chairs could be placed around two or more tables, and how should the tables be arranged?

Questions for Students

1. How can you determine the number of chairs needed if you know the number of tables? Explain your process.
2. How many tables would be needed to provide a place to sit for all of the students in your class?
3. Which arrangement do you think is best? Each table with four chairs? Pushing tables together to form a long row? A rectangular arrangement? Or is there another arrangement that you would recommend?

Teacher Reflection

• Did the context of the problems provide a high level of enthusiasm? If not, what context might be better?
• Were students able to effectively organize the data they collected and use it to identify patterns? If you taught this lesson in the future, would you supply graphic organizers to students?
• Did students use pattern blocks, the Illuminations Patch Tool, or some other manipulatives to investigate the problems? If so, how well did they work, and what would you change?
• What adjustments could be made to extend these problems for students who quickly identified the patterns?
• What additional support could be given to students who had difficulty identifying the patterns?

### Learning Objectives

Students will:

• Identify and extend a linear pattern involving the number of chairs that can be placed around a series of square tables.
• Describe linear patterns using words or symbols.

### NCTM Standards and Expectations

• Describe, extend, and make generalizations about geometric and numeric patterns.
• Represent and analyze patterns and functions, using words, tables, and graphs.
• Represent the idea of a variable as an unknown quantity using a letter or a symbol.
• Express mathematical relationships using equations.
• Identify and describe situations with constant or varying rates of change and compare them.