To help the students become more familiar with the set model for subtraction, ask them to watch as you show six pasta shapes in your hand. Tell them you are going to play "Guess How Many?" Then put your hands behind your back, transferring some, say four, of the pasta shapes to your other hand. Ask a volunteer which hand to
show, and then bring that hand forward, opening it so the pasta shapes are visible. Now ask the students to "guess" how many pasta shapes are in your other hand and then count to verify their guess. Then give the students a chance to
play this game with one another. [This activity involves a missing addend situation (6 - __ = 4) in concrete form.] Note: This concept involves abstract thinking that may be too challenging for some students. Other students may need to divide manipulatives of their own into two sets to answer the question.
Now place the students in pairs and provide them with pasta shapes and a piece of paper for a workmat. Tell them that they will be telling take-away stories and then using pasta shapes to model the stories. Remind them to record
how many they started with, how many they took away, and how many were left using either the vertical or horizontal format practiced in the previous lesson. Note: Students generally find the vertical format easier to use, so you may wish to suggest this format to the students who are more challenged by this material.
To model the process, call on a volunteer to start a story that involves a number that is 10 or less. For example, a set of seven birds were at the feeder. Ask the students to show the corresponding number of pieces of pasta on their paper plate. Then call on another volunteer to continue the story by adding a sentence that involves taking away. For example, a set of three birds flew away. Invite the students to take away the mentioned number of pasta pieces. Then ask, "How many pieces of pasta are on your plate now?" [In the example, four.] Ask the students what that number is called. [The difference.] Now call on a volunteer to record the subtraction in the horizontal format. Ask another volunteer to record the subtraction in the number sentence in vertical format. Repeat with another story until the students are comfortable with the process. Give the pairs time to tell and record at least two stories.
Another example includes the following:
Mary made 5 heart shaped cookies. She ate 2 of them. How many cookies are left?
Then tell them they will record subtraction in another way--in chart form. Distribute the Take-Away Activity Sheet, to the pairs.
Ask them to point to the column labeled “Number of Pasta Shapes.” Then ask them to read the titles of the other two columns. Call on a volunteer to make up a story problem. Guide the students in recording values in the first two columns. Ask another student to tell what should be written in the “Number Left”
column. Now have the students work together to create new entries for the chart. Encourage them to model each trio of entries with pasta shapes. After they have had time to create several rows on their charts, call the students together and ask them to share one of the rows that they have written.
Number of
Pasta Shapes |
Number
Taken Away |
Number
Left |
| 6 |
2 |
4 |
| 5 |
2 |
3 |
| 5 |
5 |
0 |
| 5 |
0 |
5 |
| |
|
|
| |
|
|
As you discuss their entries, you may wish to review the terms “take away” and “difference.” If no
student shares a row where zero is taken away, ask the class what would be recorded if you started with five pasta shapes and took zero pieces away. Encourage the students to act this operation out with their pasta shapes and record the result on their charts. Repeat with a model for a difference of 0. Prompt the students to add this entry to their chart as well.
Now ask the students to write a subtraction story problem that uses sets and record the results in two ways. When they have completed this task, encourage them to share their problem with a friend and add it to their portfolios. You may wish to display the charts in the classroom.
In the example above, a set of 2 was taken away from a set of 6.
