Transcript Multiplying Decimals

Multiplying Decimals by Decimals
1
Decimal
Review
2
Decimals Represent Fractions
3
6
Ten thousandths
Thousandths
6 . 8
Ones
Hundredths
8/10 3/100 6/1000 1/10,000
Tenths
6
1
Notice that every decimal ends with “ths”.
3
Which decimal place value is missing?
Tenths
Hundredths
Thousandths
Ten Thousandths
Ten Thousandths
6 . 8
3
6 2
4
Which decimal place value is missing?
Tenths
Hundredths
Thousandths
Ten Thousandths
Thousandths
6 . 8
3
6 2
5
Which decimal place value is missing?
Tenths
Hundredths
Thousandths
Ten Thousandths
Tenths
6 . 8
3
6 2
6
Which decimal place value is missing?
Tenths
Hundredths
Thousandths
Ten Thousandths
Hundredths
6 . 8
3
6 2
7
How to
Multiply
Decimals
8
3.15 x 1.9
Round to the nearest whole number to estimate answer
3x2 =6
So our answer should
be somewhere around 6
Do the multiplication (just ignore the decimals for now)
1
4
315
x 19
28 3 5
315 0
5 98 5
9
Our estimate
Answer before doing
anything with decimals
6
5. 9 8 5
Let’s look at another example
to see if we can come up with a
rule…..
10
2.7 x 1.1
Round to the nearest whole number to estimate answer
3x1 =3
So our answer should
be somewhere around 3
Do the multiplication (just ignore the decimals for now)
27
x 11
27
27 0
2 97
11
Our estimate
Answer before doing
anything with decimals
3
2 .9 7
One more example…..
12
4.125 x 1.8
Round to the nearest whole number to estimate answer
4x2
=8
So our answer should be
somewhere around 8
Do the multiplication (just ignore the decimals for now)
1
2 4
4125
x 18
33 0 0 0
41 2 5 0
74 250
13
Our estimate
Answer before doing
anything with decimals
8
7.4 2 5 0
Ok, lets look at all of our
examples…..
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Problem
Number of
Decimal
Places in
problem
Answer
Number of
Decimal
Places in
answer
3.15 x 1.9
3
5.985
3
2.7 x 1.1
2
2.97
2
7.4250
4
4.125 x 1.8 4
Look at the number of places after the
decimal point in both the problem and
the answer……
15
How to multiply decimals
1
2
3
Count
the
#
of
digits
Estimate
Multiply in decimal places in
the
(ignore the factors. That’s the
products
decimal) number of decimal
first.
places in the product.
123
4.235
4.235 4 4.235
4
x 1. 6
x 1.8 2 x 1. 6
7
6230
76230
8
1234
.
16
Copyright © 2013 by Monica Yuskaitis
Solve this problem
3 2
9. 8 7
x .4
3.9 4 8
17
Solve this problem
5. 2 3
x .21
5 23
+104 6 x
1 .0 9 8 3
18
Solve this problem
2 3
2 2
3 4
7 6. 8
x 1 4. 3 5
1 38 40
2304x
+ 3072xx
3 3 4 .0 8 0
19
Problem
Solving
20
George lived 13.7 miles from his
school. He walked 1/2 (.5) of the way.
How far did he walk?
13.7 miles
21
.5 of 13.7
•
The first thing to remember is “of”
means multiply in mathematics.
of = x
22
.5 x 13.7
13
1 3.7
x .5
6. 8 5 miles
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