Display the NCTM Electronic Abacus and demonstrate how to create a set of up to 10 counters on the top line (by clicking on the beads on the left).
Then select a student to count one bead away from the set that
you made. (By clicking on the beads, they will move from the left side
of the abacus to the right side.) Call on a volunteer to write on the
board the number sentence that shows what happened. (Example:
5 – 1 = 4.) Repeat and have other students take turns counting away one
bead and writing the number sentence. Then repeat counting back more
Give each student 10 to 12 pasta shapes and a paper plate. Then tell
a story that involves counting back one such as, there were six pieces
of pasta on the plate and Jerry took one. How many are left? Ask the
students to model with pasta shapes what is happening as they listen to
the story. Then ask them to write the numeral for the whole set, for
what was taken away, and for what was left. Repeat with other stories.
Check to see that all the students are able to write the numerals that
correspond to each set.
Next, hold up an index card displaying a number (for example, 4),
and ask the students to make a set with that many pasta shapes, place
them on the plate, and record the number of pasta shapes on the plate.
Then ask the students to count three pieces of pasta from the plate and
record the number remaining on the plate. Now write the appropriate
equation in horizontal format, and ask them to copy it. Now show
another number and ask the students to make a set with that many pasta
shapes, then call on a volunteer to tell how many pieces should be
taken away this time. As the students model the subtraction, introduce
the vertical subtraction format and ask them to also record the
subtraction in this way. Ask them to compare the two formats.
Now put the students into pairs and give each pair a number cube.
Assign one student in each group to roll the number cube twice,
recording both numbers that come up. Tell the other student to model a
subtraction by counting away the smaller number from the larger number.
Tell the first student to record the subtraction in two different ways.
Have them reverse their roles and repeat several times.
When the students are ready, ask each pair to read one of their
subtraction examples and show the two ways they recorded it. Then ask
the students to draw one subtraction situation of their choosing and
record it in both vertical and horizontal formats.