1 Multiplying Decimals by Decimals. 2 DecimalReview.
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Transcript of 1 Multiplying Decimals by Decimals. 2 DecimalReview.
1
Multiplying Decimals by Decimals
2
DecimalDecimalReviewReview
3
Decimals Represent Fractions
6 . 8 3 6 1
Ten
ths
Hun
dred
ths
Tho
usan
dths
6 8/10 3/100 6/1000 1/10,000
Notice that every decimal ends with “ths”.
One
s
Ten
thou
sand
ths
4
Which decimal place value is missing?
Ten ThousandthsO
nes
Ten
ths
Hun
dred
ths
6 . 8 3 6 2
Ten
Tho
usan
dths
Tho
usan
dths
5
Which decimal place value is missing?
ThousandthsO
nes
Ten
ths
Hun
dred
ths
6 . 8 3 6 2
Ten
Tho
usan
dths
Tho
usan
dths
6
Which decimal place value is missing?
TenthsO
nes
Ten
ths
Hun
dred
ths
6 . 8 3 6 2
Ten
Tho
usan
dths
Tho
usan
dths
7
Which decimal place value is missing?
HundredthsO
nes
Ten
ths
Hun
dred
ths
6 . 8 3 6 2
Ten
Tho
usan
dths
Tho
usan
dths
8
How to How to MultiplyMultiply
DecimalsDecimals
9
3.15 x 1.9 3.15 x 1.9 Round to the nearest whole number to estimate answerRound to the nearest whole number to estimate answer
3 x 2 3 x 2
= 6 = 6 Do the multiplication (just ignore the decimals for now)Do the multiplication (just ignore the decimals for now)
315315 x 19 x 19 5 5
4 4
3 3
1 1
28 28 5 5
0 0
1 1
3 3 5 5
8 8
9 9
5 5
So our answer should So our answer should be somewhere around 6be somewhere around 6
10
Our estimate Our estimate
5 9 8 55 9 8 5
Based on our estimate, where do
Based on our estimate, where do
you think the decimal should go?
you think the decimal should go?
Answer before doing Answer before doing
anything with decimalsanything with decimals
6 6 . .
Let’s look at another example to Let’s look at another example to see if we can come up with a see if we can come up with a
rule…..rule…..
11
2.7 x 1.1 2.7 x 1.1 Round to the nearest whole number to estimate answerRound to the nearest whole number to estimate answer
3 x 1 3 x 1
= 3 = 3 Do the multiplication (just ignore the decimals for now)Do the multiplication (just ignore the decimals for now)
2727 x 11 x 11 7 7
2 2 7 7
0 0
2 2 7 7
9 9
2 2
So our answer should So our answer should be somewhere around 3be somewhere around 3
12
Our estimate Our estimate
2 9 7 2 9 7
Based on our estimate, where do
Based on our estimate, where do
you think the decimal should go?
you think the decimal should go?
Answer before doing Answer before doing
anything with decimalsanything with decimals
3 3 . .
One more example…..One more example…..
13
4.125 x 1.8 4.125 x 1.8 Round to the nearest whole number to estimate answerRound to the nearest whole number to estimate answer
4 x 2 4 x 2 = 8 = 8 Do the multiplication (just ignore the decimals for now)Do the multiplication (just ignore the decimals for now)
41254125 x 18 x 18
0 0
4 4
0 0
2 2
0 0 5 5
0 0
2 2
1 1 0 0
5 5
2 2
4 4
So our answer should be So our answer should be somewhere around 8somewhere around 8
1 1
33 33 4 4 7 7
14
Our estimate Our estimate
7 4 2 5 0 7 4 2 5 0
Based on our estimate, where do
Based on our estimate, where do
you think the decimal should go?
you think the decimal should go?
Answer before doing Answer before doing
anything with decimalsanything with decimals
8 8 . .
Ok, lets look at all of our Ok, lets look at all of our examples…..examples…..
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ProblemProblem
7.4250 7.4250
Number of Number of Decimal Decimal Places in Places in problemproblem AnswerAnswer
4.125 x 1.84.125 x 1.8
2.7 x 1.12.7 x 1.1
3.15 x 1.93.15 x 1.9
2.97 2.97
5.985 5.985
Number of Number of Decimal Decimal Places in Places in answeranswer
33 33
22
44
22
44
Look at the number of places after the Look at the number of places after the decimal point in both the problem and the decimal point in both the problem and the answer……answer……
Copyright © 2013 by Monica Yuskaitis16
x4.235
61. x4.235
67 6230
1. 4.235 1.8x
How to multiply decimals
Estimatethe
productsfirst.
Count the # of digitsin decimal places in
the factors. That’s thenumber of decimal
places in the product.
1 2 3
Multiply (ignore
decimal)
4
2 8 76230
123
4
.1234
17
Solve this problem
9. 8 7. 4x
8
2
4
3
3 9.
18
Solve this problem
5. 2 3. 2 1x
3256 x41 0 +
3890.1
19
Solve this problem
7 6. 84. 3 5x
0
4
4
3
3 84 x
2
0
2
2 3x x273 0
32
+080
1
4
1
33 .
20
ProblemProblemSolvingSolving
21
George lived 13.7 miles from his school. He walked 1/2 (.5) of the way. How far did he walk?
13.7 miles
22
.5 of 13.7• The first thing to remember is “of”
means multiply in mathematics.
of = x
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.5 x 13.7
1 3.7.x5
3
8
1
6 .
5
miles