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Multiplying Decimals
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To Multiply:
• Place the number with more digits on top.• Multiply• Count the number of decimal places (from the
right) in the question.• Make sure that you have the same number of
decimal places in your answer.
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Example: 5.63 x 3.7
5.633.7x
1
2
4
4
39098
1
16+13
1
8
1
0
1
2
two
one
three.Next slide
2.3 x 1.4
When multiplying these two numbers together, it is easier to put them into a grid.
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2.3 x 1.4
20 3
10
4
For now let’s cheat and pretend that it is not Decimals – just remember there are two numbers to the right in the question.
The first thing to do ispartition the numbers into tens and units.
2 decimals to the right
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2.3 x 1.4
20 3
10
4
Multiply the number abovethe box with that on the left,writing the answer inside the box,as shown below.
200 30
80 12
What goes here?
2 decimals to the right
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2.3 x 1.4
200
20 3
10
4
The next step is to add upthe rows, putting the answerin the boxes on the right.
30
80 12
230
92
What goes here?
2 decimals to the right
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2.3 x 1.4
200
20 3
10
4
Now add up the totals on theright to NEARLY give the answer.
30
80 12
230
92
322
What goes here?
2 decimals to the right
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2.3 x 1.4
200
20 3
10
4
How many numbers to the right in the question?
30
80 12
230
92
322
2 decimals to the right
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2.3 x 1.4
200
20 3
10
4
How many numbers to the right in the answer?
30
80 12
230
92
3 22.
2 decimals to the right
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Now you can work out multiplication using the grid method, you will find long multiplication easy!
This is a little different from the grid method, as the numbers are set out in columns…
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5.1 x 4.2
becomes5142x
Qwizdom question – what would your estimate be? Whole numbers only using nearest nice number?
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5.1 x 4.25142x
Remember whatwe did usingthe grid method?
The first thing to do, is multiply the tens of thetop number with the tensand units of the bottomnumber.
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5.1 x 4.25142x
Next, multiply the units of the top number by the tens and units of the bottom number.
50 x 40 200010050 x 2
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5.1 x 4.25142x
Don’t forget to write down themultiplicationnext to theanswer.
50 x 40 200010050 x 2
1 x 40 401 x 2 2
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5.1 x 4.25142x
The NEARLY final step isto add up thecolumn ofnumbers and putthe total at the bottom.
50 x 40 200010050 x 2
1 x 40 401 x 2 2
2142Qwizdom – what would the final total be?
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5.1 x 4.25142x
What is the actual answer?
50 x 40 200010050 x 2
1 x 40 401 x 2 2
2142Next slide
Multiplication Properties
Commutative Property of Multiplication: Factors can be multiplied in any order.
Example: 12 x 5 = 5 x 12
Associative Property of Multiplication: Factors can be grouped in any way.
Example: (2 x 3) x 5 = 2 x (3 x 5)
Identity Property of Multiplication: The product of any number and one is that number.
Example: 18 x 1 = 18Next slide
LO: To be able to multiply with decimals
I understand this idea fully.
I have some understanding but I need more practice.
I don’t understand this idea at all and need to work again on this unit of work.
A
B
C End of slide showEnd of slide show
