To review the concept of subtraction, read Ten Monsters in the Bed,
or sing the song “Ten in the Bed.” Call on volunteers to demonstrate
with connecting cubes what is happening and to write or draw pictures
of the related addition and subtraction sentences for each part—for
example, 10 – 1 = 9 and 9 + 1 = 10.
Then call out a difference (for instance, “5”) and have each
student show you the meaning of the subtraction sentence by making two
connecting cube trains that have the stated difference. Examples
include trains of 7 and 2, 3 and 8, and 4 and 9. Next have the students
add cubes in a third color to the shorter train and describe the
relationship between the trains in as many ways as they can using
subtraction sentences and then related addition sentences. [Examples
include 7 – 2 = 5, 7 – 2 = 2, 5 + 2 = 7, and 2 + 5 = 7.] When the
children seem comfortable with this exercise, distribute connecting
cubes in three colors to pairs of children and have them take turns
making trains and writing equations. Encourage the students to make two
trains, each 7 cubes long, and write the related addition and
subtraction sentences: 7 – 7 = 0 and 0 + 7 = 7.
Then call the class together and ask a volunteer to make two
trains and write the four number sentences (two addition and two
subtraction) that the trains suggest. You may wish to repeat this with
other volunteers. Invite one of the students to make a train with three
connecting cubes of one color and three of another and write the
related addition and subtraction sentences. Note that there will be
only one addition and one subtraction sentence.
Finally, ask the students to write a set of number sentences about one of the trains that they made.
Display the online Calculators and Hundred Boards:
Displaying Number Patterns tool and allow groups of students to take turns finding differences using the online calculator.
In addition, children can use actual calculators to find several
pairs of numbers less than 12 that have a difference of “2” and then
record the subtraction sentences. Ask students to repeat the exercise
with another difference, such as “3.” Ask students to explain the
number sentences, how and why they are alike and different.
Students should record the fact families they have identified in this lesson on the Fact Families Activity Sheet.