Before the beginning of class, make enough copies of the Multiplication Model Activity Sheet for each student. Also, a copy of the Decimal Multiplication Overheads will be necessary for the teacher.
Group students in pairs and distribute base ten blocks to
each group. Tell students they will be using base ten blocks to model decimal
multiplication. They will begin by modeling multiplication of two whole
numbers: 34 ×
23. Ask students to first make the number 34 using the traditional
representation of a rod as "10" and a unit as "1" and to
arrange their formation on the left of their Multiplication Model Activity Sheet. Be sure to tell
students that all the rods should be together and all the units should be
together (instead of alternating). Show Figure 1 Overhead to demonstrate this.
Multiplication Model Activity Sheet
Decimal Multiplication Overheads
Next, have students construct the number 23 on the same
array, placing the blocks above the top line this time. Tell students that if
their rods representing 34 are at the top of the left side, then the rods
representing 23 should be on the left side of the top row; if the rods for 34
are at the bottom of the left side, then the rods for 23 should be on the right
side of the top row. Point to the spots on the overhead, so students are better
able to visualize this. Ask students to display the rods first, and then, display the units (to be consistent with place value). As students complete this step, display OH- Figure 2 on
the overhead projector.
The next step is to complete the area model by filling in
the rest of the model, using blocks that match the length of the blocks that
are on the left and at the top. After students complete this step, ask a
student to share their model with the class by sketching what they did on
After completing this model, tell students it is time to
determine the product of this multiplication problem. Sample guidance: First,
count the number of units. Because there are 12 of them, you are able to
combine 10 of them together to make a rod. This leaves two units. Now count the
number of rods. There are 18 rods, including the one you made by combining
10 units. Ten rods are equal to one flat. Therefore, combine 10 of the 18 rods,
and exchange them for a flat. This leaves eight rods. Now count the number of
flats. Including the one that you just made from the 10 rods, there are 7
flats. The 7 flats plus the 8 rods plus the 2 units is equal to 782, therefore,
23 = 782.
Depending on the students' work, you may choose to create additional examples.
Ask students to think about changing the value of the units,
rods, and flats. Ask, "Suppose each flat was worth 1 whole, what would
each rod be worth?" [1/10] Using the same figures as before, show Figure 1
and ask, "What is the value of the blocks on the left now?" [3.4] Show
Figure 2 and ask, "What is the new value of the top row?" [2.3]
Then show Figure 3 again, and have students determine the value of the base ten
blocks using the new values. As students are computing, ask, "What is the
value of each single unit?" [1/100] As students are working, remind them
to think about the relationship between the initial value of a flat and the new
value of a flat.
As students finish, ask them what the product is using the
new values. [7.82] Ask, "How does this value compare with the product of
34 times 23?" [It is 1/100 the value.] For advanced students, you may also
ask, "Why is the value of 3.4 × 2.3 equal to 1/100 the value of 34 ×
23?" [Because both numbers are equal to 1/10 the previous value, and 1/10 × 1/10
Begin the next class period with a review of the previous
lesson. You can check for understanding using a similar problem, with
different digits: "How can you find the value of 6.1 ×
7.2?" [You know that 6.1 is 1/10 of 61 and 7.2 is 1/10 of 72. The product
of these two decimals is 1/100 the product of the whole numbers. Because 61 × 72
is 4,392, the product of 6.1 × 7.2 is 4,392/100, or 43.92.]
Then show OH- Figure 3 on the overhead projector. Using the
value of 1 flat = 1 whole, ask students to determine the following:
- The value of the far-left
column (the first factor) [1.6] State that this value is the first factor.
- The value of the top row (the second factor) [4.4] State that this value is the second factor.
- The value of each
- The total value (the product of the two factors) of the
base ten blocks [7.04]
Now ask, "Can you model the base ten blocks to
determine two different factors that also have a value of 7.04?" [Yes, 3.2 ×
2.2 and 6.4 ×
1.1.] Allow students to rearrange the blocks again to see if they can find more factors. [8.8 × 0.8, 17.6 × 0.4] Be sure to reinforce the concept that the product being modeled by the blocks must be a rectangle.
Now ask students to pair up. Provide them with base ten
blocks. Ask them to configure three instances of decimal multiplication that
yield the same decimal product. For each instance, they should do the
- Draw the configuration.
- Label the factors and
- Redraw using the same
product to determine two different factors.
After the activity, conduct a class discussion by asking,
"Are there certain types of factors you know will allow you to redraw them into
two new factors?" "What types of factor pairs will not let you redraw them into two new factors?" [When both factors are prime numbers.] Students should be able to identify the characteristics that
allow this to happen, namely that at least one of the factors of the first
instance should be divisible by 2, 3, or some other reasonably small prime
1. Have students determine which multiplication will yield a
4.2 or 2.7 × 3.9
Have students support their solution by using and tracing the base ten blocks.
[2.7 × 3.9, because 27 × 39 = 1053, which is
greater than 1050, the product of 25 × 42]
2. Have students write a real-world story about a decimal
multiplication problem that they have created with the base ten blocks. The
story should contain each of the different numbers that are involved with the
rectangular construction. For example, stories could involve painting a
rectangular wall in a room, or carpeting a rectangular floor in a room, and the
dimensions of the two-dimensional space are the given decimals.
3. Ask students to draw their base ten blocks on figure paper to demonstrate student understanding. Note which students have mastered the skill.
- Have students explore
what happens if the flat is equal to 1/10 and the rod is equal to 1/100. What is the value of a unit? [1/1000.] How is a product
using these values related to the product when a flat is equal to 1 and a rod
is equal to 1/10? [The product, when a flat is equal to 1/10, is a hundreth of the product (when the flat is equal to 1).]
- Have students work with factor trees to develop different
models from the original model. For example, if they construct a 1.2 by 3.6
rectangle, the product will be 4.32. To make a model of a different shape,
instead of moving the different shapes around, students should make a factor
tree for the whole number 432. This moves the decimal two places, or multiplies
4.32 by 100. The prime factorization of 432 is 2 × 2 × 2 × 2 × 3 × 3 × 3. Now
students should divide these prime factors into two groups. For example, they
can place 2 × 2 in one group and the remaining factors 2 × 2 × 3 × 3 × 3 in the
second group. The product of the first group is 4, and the product of the
second group is 108. Returning to decimals, students can move the decimal place
one place to the left in each factor (which equals two places total) to reverse
what they did at the beginning. The 4 can be changed to 0.4, and the 108 can be
changed to 10.8, and 0.4 ×10.8 = 4.32. In this example, a 0.4 by 10.8 rectangle
can be created with the base ten blocks. Note that 4 × 1.08 = 4.32, also, but
students may find that more difficult to represent with base ten blocks.