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Models and Pictorials
Warm-up
Say Cheese!
Powerpoint
Multiplying Decimals
Guided Practice
Ice Cream Shop
Say Cheese!
(Warm Up)
1. Mr. Flanagan buys a 2½ pound wheel of
cheese. His family eats 1⅓ of the wheel. How
much cheese have they eaten?
2 12
x
113 = 52
1
4 2 10
x3 = 3 =
3 13 pounds
2. If each person in North America throws away
3⅔ pounds of garbage each day, how many
pounds of garbage does each person throw
away in a week?
3 23
x 7=
11
3
x
2
7 77
1 = 3 =25 3
pounds
Decimal Times
We can use what we learned about
multiplying fractions to help us understand
multiplying decimals. Use area models to
multiply 0.5 x 0.3.
0.3
= 0.15
X
0.5
=
X
Adapted from CMP2, Bits &
Pieces III
Decimal Times
Use area models to multiply 0.9 x 0.4.
0.9
X
0.4
=
=
X
Adapted from CMP2, Bits &
Pieces III
0.36
Decimal Times
Use area models to multiply 0.7 x 0.8.
0.7
X
0.8
=
=
X
Adapted from CMP2, Bits &
Pieces III
0.56
Decimal Times
What is the relationship between the products
in Set A and the corresponding products in
Set B.
Set A
Set B
0.5 x 0.3 = 0.15
5 x 3 = 15
0.9 x 0.4 = 0.36
9 x 4 = 36
0.7 x 0.8 = 0.56
7 x 8 = 56
The products in Set B are greater than the
products in Set A, but they have the same
digits.
Adapted from CMP2, Bits &
Pieces III
Decimal Times
What conclusion can we draw about
multiplying decimals based on this
relationship?
Whether we are multiplying whole
numbers or decimals, you start with
multiplying the whole numbers. For
example: 0.5 x 0.3, you start by
multiplying 5 x 3.
Adapted from CMP2, Bits &
Pieces III
Decimal Times
So far we only dealt with decimals in the tenths place.
Let’s use our calculator to explore what happens
when we multiply decimals in the hundredths and
thousandths place.
What pattern do you notice?
0.05 x 0.03 = 0.0015
0.05 x 0.003 = 0.00015
0.005 x 0.003 = 0.000015
Possible answer: More digits behind the decimal in
the problem results in more digits behind the decimal
in the answer; or the total number of digits behind the
decimal in the problem is the same as the number of
digits behind the decimal
in the answer.
Adapted from CMP2, Bits &
Pieces III
Decimal Times
With your partner, write an algorithm for
multiplying any two decimal numbers.
To multiply decimals, multiply the digits as if
they were whole numbers; then count the
number of digits behind the decimal point in
both factors. That sum tells you how many
digits are behind a the decimal point in the
product.
Adapted from CMP2, Bits &
Pieces III
Ice Cream Shop
(Guided Practice)
1. Sweety’s Ice Cream Shop sells ice cream
by the weight. They charge $2.95 per
pound. Suppose your dish of ice cream
weighs 0.42 pounds. How much will your
ice cream cost?
$2.95 x 0.42 = 1.239
(you will be charged $1.24)
Ice Cream Shop
(Guided Practice)
2. Aaron plans to buy new flooring for his
rectangular office. His office is 7.9 meters
by 6.2 meters.
A. How many square meters of floor space
does his office have?
7.9 m x 6.2 m = 48.98 sq. m
B. Suppose flooring costs $5.90 per square
meter. How much will the new flooring
cost for Aaron’s office?
48.98 sq. m x $5.90 = $288.98