Multiplying Decimals NS3

1
Multiplying DecimalsNS3

• I can multiply decimals.

2
Multiply Decimals

• Step 1 --- Ignore the decimal
• Step 2 --- Rewrite the problem
• Step 3 --- Multiply the problem
• Step 4 --- Count the decimal places
• Step 5 --- Move decimal that many places in your
  answer

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Guided Practice

• 4 . 5 6
• X 0 . 3 9

4
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.
0.7 ? 0.3 0.21
Numbers
5
Multiplying Decimals
5.82
2 decimal places
0.41
2 decimal places
5 8 2
2 3 2 80

4 decimal places
2.3 8 6 2
1
2
3
4
6
Example 5.63 x 3.7
5.63
x
3.7
7
Multiplying Decimals
6.45
2 decimal places
18
0 decimal places
5160
6450

116.10
2 decimal places
ANSWER
6.45 ? 18 116.1
Because 6 ? 20 120, the product is reasonable.
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Multiplying Decimals
1.273
6.45
3 decimal places
2 decimal places
0.06
18
2 decimal places
0 decimal places
5160
7638
0.0
5 decimal places
645

116.10
2 decimal places
ANSWER
ANSWER
6.45 ? 18 116.1
1.273 ? 0.06 0.07638
CHECK
Because 6 ? 20 120, the product is reasonable.
Because 1 ? 0.06 0.06, the product is
reasonable.
9
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 l w
Write formula for area of a rectangle.
l w
2.5
(0.5)
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.
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Showdown!!!!

• 3. 2 x .006
• .012
• 4. .66 x 2.52
• 1.6632

• 1. .31 x .7
• .217
• 2. .9 x .03
• .027