To set the stage, you may wish to read another of the counting back books listed in the Bibliography of Children's Counting Books. One appropriate book includes Ten, Nine, Eight, and Ten
Monsters in a Bed. If you prefer, you may choose to have the students act out such subtraction situations as the following:
Call seven children to the front of the room, then roll a die to decide how many will return to their seats. Have a volunteer record on the board the subtraction equation the children acted out.
Next, model a subtraction problem in which two sets are compared. For example: Jody’s shirt has 5, buttons and Sandy’s shirt has 3 buttons.
What questions can we answer about the shirts?
- How many more buttons does Jody’s
shirt have?
- How many fewer buttons are on Sandy’s shirt?
Now have the students pose and answer similar questions. Then have the children write a subtraction story problem in which a set of 3 and a set of 4 are compared. Encourage them to model the problem with cubes and to share their problems with the class. Next, encourage the students to make up another problem, this time showing the comparison of a train of 5 with a set of 7. When the students are ready, call on individuals to share their problems. You may wish to suggest that they record in pictures and in equation form one of the comparing problems for their
portfolios.
Display a large piece of chart paper where all the students can see it. Point out the columns you have labeled “Cubes in Shorter Train,” “Cubes in Longer
Train,” and “Difference.” Have the children follow along on the Short and Long Cube Trains Activity Sheet.
Display a train of 6 connecting cubes in one color and a train of 4 cubes in another color. Ask the children how the trains could be compared. Then have them dictate the entries for each column. If children have difficulty comparing, have them add cubes in a third color to the shorter train until the trains are the same length. Then suggest that the students count the
cubes they added.
Provide pairs of children with connecting cubes and a piece of paper for a workmat. Tell them that they will be making trains and then comparing them. Ask them to record the number of cubes in each train and how many more cubes are in the longer train. Now have them work in pairs to create new entries for the chart. As the students identify differences, call on volunteers to enter their findings on the class chart. Allow the children time to make several entries, then call them together and review the terms “compare” and
“difference.” Finally ask what would be recorded if both trains had 7 cubes. [7,7, 0] Prompt them to add other such entries.
To help the students to become more familiar with the set model for comparison subtraction, tell them you are going to teach them a game, “So Many More.” Show a train of cubes (any number greater than 1 will work.) Then roll a number cube and ask how many cubes will be on a train with that many more cubes.
Make a train to verify their responses. Next ask a volunteer to start with the same number, roll the die, and make a train with that many cubes. Now compare the trains. The player with the longer train makes a tally mark. Finally, give the students a chance to play five rounds with a friend. The winner will be the child in the pair with the most tally marks.
