Transcript Multiplying Decimals

MULTIPLYING DECIMALS
Lesson 3.4
Using Base Ten Blocks


The flat represents 1, the rod represents 0.1 and
the small cube represents 0.01
By using Base Ten Blocks to make a rectangle, you
can switch to addition instead of multiplication for
decimals.
Modeling

A rectangular park measures 1.7 km by 2.5 km.
Step 1: Build a rectangle with length of 2.5 and width
1.7
Width (1.7 km)
Length (2.5 km)
When the rectangle is
finished, count the blocks in
the rectangle.
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There should be 2 flats: 2 x 1 = 2
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There should be 19 rods: 19 x 0.1 = 1.9
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There should be 35 cubes: 35 x 0.01 = .35
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The total area will be: 2 + 1.9 + .35 = _________
But How Do I Multiply??
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1.7 x 2.5
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Pretend there are no decimals.
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17 x 25
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Multiply as normal.
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Then add the decimal after.
But Where Do I Put The Decimal?
1.7 x 2.5
1.7
x 2.5
4.25
Ask yourself, “How many numbers
are after the decimal?” = 1
Ask yourself, “How many numbers
are after the decimal? = 1
There are 2 numbers total after the
decimal point. Start at the end of
the number, then, move two
numbers to the left. Where you
stop the decimal goes.
How Do I Multiply 2 Two Digit
Numbers?
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A brief reminder...
13
x 12
26
190
216
1) Multiply 3 x 2 first. Put the answer below the 2. (6)
2) Then, multiply 2 x 1. Put the answer beside the 2 and
under then 1. (2)
3) Then, put a zero under the 2. (0)
4) Then multiply 1 x 9. Put the answer beside the zero and
under the 1. (9)
5) Then, multiply 1 x 1. Put this answer beside the 9.
6) Add the two numbers together to get your answer.

Try these with long multiplication.
a)
14 x 21
b)
17 x 12
c)
15 x 20
Word Problems
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
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At the Farmers Market, 1 kg of grapes costs $2.95.
How much would 1.8 kg of grapes cost?
1 kg of grapes costs $2.95.
So, 1.8 kg would cost 2.95 x 1.8
Create a rectangle model (Base Ten)

2.95 x 1.8
2.95
2.0 x 1.0
0.95 x 1.0
1.8
2.0 x 0.8
0.95 x 0.8
(2.0 x 1.0) + (0.95 x 1.0) + (2.0 x 0.8) + (0.95 x 0.8)

(2 x 1) + (0.95)
+ (2 x 0.8) +(0.95 x 0.8)

2
+
0.95
+ 1.6
+ 0.76
= 5.31
Therefore, 1.8 kg of grapes would cost $5.31.
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A kg of apples costs $1.90.
How much would 2.1 kg of apples cost?
Draw the rectangle model and solve.
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
An area rug is rectangular. Its dimensions are 3.4 m
by 2.7 m.
Solve using multiplication.
Estimation

To estimate using multiplication, multiply the two
whole numbers together. (First round up or down to
the nearest whole number)
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5.3 x 2.1 = 5 x 2 = 10

A good estimation is 10.

Multiply to verify your estimation.

Use estimation, then solve using multiplication.
Remember to round up or down to the nearest
whole number)
a)
2.6 x 1.3
b)
7.2 x 4.8
c)
3.9 x 3.2
Homework
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Workbook 3.4
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Textbook Page 102 – 103 # 4, 5, 9, 10 and Reflect