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 following:
- 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
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