Call seven students to the front of the room, and then roll a number cube to decide how many more will come to form a second group. Next, ask each group to form a line so that the lines are parallel and the last person in each line is standing against the board. Then tell each student to hold hands with the student across from him
or her. Have a volunteer say how many more students were in the larger group, and then record the subtraction equation where all the students can see it.
Continue the lesson by reading a book that features fish, such as Swimmy, by Leo Lionni.
To help the students become more familiar with the set meaning for comparison subtraction, tell the students you are going to compare sets of crackers. Show a plate of fish-shaped crackers. Then roll a number cube and ask how many there will be in a plate with that many more fish-shaped crackers. Make a second plate with that many crackers to verify the students' responses. Then encourage them to write the subtraction equation that would be used to compare the two sets.
Repeat this procedure several times. Then, if necessary, review the terms "addend," "compare," and "difference." Ask what the addends and difference would be if one plate has four crackers and the other has six? [4, 6, 2] Ask what the
addends and difference would be if both plates had seven fish-shaped crackers [7, 7, 0]. Prompt the students to create other such entries.
Next, ask the students to watch as you solve a subtraction problem in which two sets are compared. For example, you could say that Jen’s plate has five
crackers and Sally’s plate has three crackers. Then create two sets where everyone can see them, surrounding one set with red yarn and the other with blue yarn. Ask the students to imagine the yarn loops represent plates. (If you prefer, you can use red and blue plates for the demonstration and for the student activity.)
Then ask questions such as the folowing:
- What comparing questions can we ask and answer about the plates?
- How many more crackers are on (student's name) plate?
- How many fewer crackers are on (another student's name) plate?
Now give each student two lengths of yarn and some crackers. Have the students pose comparison situations, model them, and answer similar questions.
When the students are ready, present a
subtraction story problem in which a set of three and a set of four are compared. Demonstrate how to make a horizontal bar graph that will allow the students to compare the data. Next, guide the students through the solution of another
problem, this one showing the comparison of a plate of two crackers with a plate of three crackers. (For example, Meg had three fish and Pete had two fish. How many more fish did Meg have?) Then ask the students to record the sets in a bar graph.
When they are ready, call on the students to share their problems and the graph. You may wish to suggest that they record a comparison in pictures, as a bar graph, and in an equation for their portfolios.
