Download Multiplying Decimals

Multiplying Decimals
I can multiply decimals
To Multiply:
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You do not line up the factors by the decimal.
Instead, place the number with more digits on
top.
Line up the other number underneath, at the
right.
Multiply
Count the number of decimal places (from the
right) in each factor.
Use the total number of decimal places in your
two factors to place the decimal in your product.
Multiply Decimals
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Step 1 --- Ignore the decimal
Step 2 --- Rewrite the problem
Step 3 --- Multiply the problem
Step 4 --- Underline all numbers to the right
of decimal(s) in original problem
Step 5 --- Move decimal that many places left
in your answer
Steps 1 & 2
4.56
X 0.39

Ignore decimals and
rewrite the problem
456
x39
Step 3
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Multiply problem without decimals
456
x 39
4104
+13680
17784
Step 4
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Underline numbers to the right of decimals in
original problem
4. 5 6
x 0. 3 9
---------------
( 4 places)
Step 5
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o
o
o
o
o
Move decimal that many places left in answer
456
x
39
4104
+13680
17784
( 4 places)
Multiplying Decimals
I can multiply decimals
Multiplying Decimals
You can use graph paper to model
decimal multiplication. Here is a
model of the product of 0.7 and 0.3.
0.7
0.3
1
Because 21 of the 100 squares are
shaded, 0.7  0.3 = 0.21.
1
Multiplying Decimals
Words
Multiply decimals as you would whole
numbers. Then place the decimal point in the product.
The number of decimal places in the product is equal to
the sum of the number of decimal places in the factors.
Numbers 0.7  0.3 = 0.21
Multiplying Decimals
EXAMPLE
1
Multiplying Decimals
5.82
0.41

––––––
582
2 3 2 80
–––––––
2.3 8 6 2
1234
2 decimal places
2 decimal places
+
–––
4 decimal places
Example: 5.63 x 3.7
1
4 2
5.63
x 37
39 4 1
+ 168 9 0
20.8 31
.
two
one
1 1
1
three
Multiplying Decimals
EXAMPLE
2
Multiplying Decimals
6.45
 18
––––––
5160
6450
––––––
116.10
2 decimal places
+ 0 decimal places
–––
2 decimal places
After you place the decimal
point, you can drop any zeros
at the end of an answer.
ANSWER
6.45  18 = 116.1
CHECK
Because 6  20 = 120, the
product is reasonable.
Multiplying Decimals
EXAMPLE
2
Multiplying Decimals
6.45
 18
––––––
5160
645
––––––
116.10
2 decimal places
1.273
+ 0 decimal places
0.06

–––––––
–––
0.07638
2 decimal places
6.45  18 = 116.1
+ 2 decimal places
–––
5 decimal places
Write a zero before the 7 as a
placeholder so that the number
has 5 decimal places.
After you place the decimal
point, you can drop any zeros
at the end of an answer.
ANSWER
3 decimal places
ANSWER
1.273  0.06 = 0.07638
CHECK
CHECK
Because 6  20 = 120, the
product is reasonable.
Because 1  0.06 = 0.06,
the product is reasonable.
Multiplying Decimals
EXAMPLE
3
Multiplying Decimals to Find Area
Central Park Central Park is a rectangular park about 2.5 miles long and
about 0.5 mile wide. What is the area of Central Park?
SOLUTION
A = lw
Write formula for area of a rectangle.
= 2.5(0.5)
l w
Substitute 2.5 for l and 0.5 for w.
= 1.25
Multiply.
ANSWER
The area of Central Park is about 1.25 square miles.
Showdown

.31 x .7 =
 .217
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2 x .006 =
 .012
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.9 x .03 =
 .027
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.66 x 2.52 =
 1.6632