## Materials Needed

You’ll need a few things before you get started with this lesson plan to teach students addition and subtraction.

- Base-ten blocks

- Wax paper

- Cheese cubes

- Plastic knives

## Description of Activity

Use the base-ten blocks to model numbers such as 251 and 372. Suggest that you would like to try substituting a cube of cheese for one centimeter block used to model the units of one. Model other numbers such as 586 of 144 with the cheese. Tell students that you are planning to cut one of the little cubes into 10 parts. (Do this so that everyone can see or in small groups. Try to get the 10 pieces as equal as possible.) Do you still have 144? (Yes) Give 4 students a tiny piece. Now you have 143 and 6/10. Show students that this number can be represented in a variety of ways: with words (two hundred forty-three and six-tenths), blocks, as a mixed number (143 and 6/10), and as a decimal number (143.6).

Use the base-ten blocks and the tiny tenth pieces to model other numbers, having students write the number on paper (while one student writes on the board) the names of these numbers in words, as mixed numbers, and as decimal numbers.

Give each group of students wax paper to work on, a plastic knife, and 4 cubes of cheese per group. Have students cut one cube into ten pieces. The fact that they are small will help students remember that tenths are parts of one. Ask students to take turns being the leader and naming numbers less than four to model (2.3, 1.7, 3.5, etc.). Students should model the numbers and write them in words, as mixed numbers, and in decimal form.

When you are ready to end the lesson, give the entire group these directions to dispose of the manipulatives. Keep in mind the primary purpose is to model decimal numbers and relate the models to the manner in which they are recorded.

- Show me 1.2 cubes of cheese. Remove these. What do you have left? (2.8)

- Show me 1 cube of cheese. In a different place show me 0.4 of a cube. Which is greater, 1 or 0.4? (1)

- Remove 0.4. What is left? (2.4)

- Remove 2. What is left? (0.4) Write how many cubes are left.

## Assessment

Check that students can relate the amount of cheese to decimal numbers and add and subtract the cheese cubes.

## Extensions

- Have students write about decimals to give you information about their understanding and to give them practice expressing mathematical ideas in narrative form. Some possible questions to answer are: Explain what a decimal number is? Why would you prefer to have 2 cheese cubes rather than 0.8 cheese cubes? What does 0.4 mean?

- Discuss how decimal numbers are used in science.

- Instead of using cheese cubes and plastic knives, this activity can be completed using paper tenth strips and scissors.