Multiplying and Dividing Decimals Now

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Multiplying and Dividing
Decimals
BEFORE
Now
WHY?
You multiplied and divided
integers and fractions.
You’ll multiply and divide
decimals.
So you can find how many balloons
you can buy, as in Ex. 24.
In the Real World
Word Watch
Rafting You travel downstream in a raft at a rate of about 4.3 miles per
hour. How far will you travel in 2.5 hours?
Multiplying Decimals
Words Multiply decimals as you do whole numbers. Then
place the decimal point. The number of decimal places in the
product is the total number of decimal places in the factors.
Numbers
2.25
2 places
EXAMPLE
1
8.9
1 place
20.025
3 places
Multiplying Decimals
To find how far you travel in the problem above about rafting, substitute
the given values into the distance formula. Distance rate p time, so
distance 4.3 p 2.5.
4.3
2.5
215
86
10.75
1 decimal place
1 decimal place
2 decimal places
A number’s leading digit is its leftmost nonzero digit. To check that a
product is reasonable, round each factor to its leading digit and multiply.
4.3 p 2.5 Round factors to leading digit.
1. 7.39 p 2.1
4 p 3 12 ✓
2. 19.62 p 5.07
Lesson 5.7
3. 1.13 p 0.04
4. 0.85 p (8)
Multiplying and Dividing Decimals
251
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Dividing Decimals
Words When you divide by a decimal, multiply both the
divisor and the dividend by the power of ten that will make
the divisor an integer. Then divide.
5.5
Numbers 2.751
5
.1
2
5
Multiply by 100.
2751
5
1
2
.5
EXAMPLE
2
Dividing Decimals
To find the quotient 60.102 6.3, multiply the divisor and dividend
by 10. Move the decimal points 1 place to the right.
6.36
0
.1
0
2
Move decimal points.
636
0
1
.0
2
Then divide.
9.54
0
1
.0
2
636
✓Check To check that the quotient is reasonable, round the quotient
and the divisor to the leading digit. Then multiply. The result should be
close in value to the dividend.
9.54 p 6.3
EXAMPLE
3
Round.
10 p 6 60 ✓
Using Zeros as Placeholders
To find some quotients, you may need to use zeros as placeholders.
Placeholder in Dividend
Placeholder in Quotient
6 1.2
0.0126 1.8
.0
1.26
Zero as
placeholder
5
0
126
60
0
252
Chapter 5
0.007
180
.1
2
6
126
0
Zeros as
placeholders
Find the quotient.
5. 1.6 0.04
6. 0.632 0.79
8. 4.365 (4.5)
9. 0.3744 1.56
Rational Number Operations
1.80
.0
1
2
6
7. 13 (0.65)
10. 0.0108 2.7
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INTERNET
Exercises
eWorkbook Plus
CLASSZONE.COM
More Practice, p. 731
1. Vocabulary Copy the division problem. Use
the words quotient, dividend, and divisor to
label each number.
_?_
_?_
0.8
.2
97
_?_
2. 7.8 p 2.6
3. 3.75 p 0.4
4. 13.2 1.1
5. 0.5 1.25
6. Guided Problem Solving A mother rhinoceros weighs 3600 pounds.
Her baby weighs 0.38 of her weight. How much does the baby weigh?
1 Write a verbal model to describe the problem.
2 Substitute the given values and solve.
Practice and Problem Solving
HE L P
with Homework
Example
1
2
3
Exercises
7–22, 23, 25
7–22, 24, 26
7–22
Online Resources
Find the product or quotient.
7. 25 p 0.2
8. 2.4 p 0.3
9. 8.2 p 0.7
11. 4.8 1.2
12. 4.9 0.07
13. 5 (0.1)
15. 5.41 p 0.35
16. 0.57 0.38 17. 4.844 0.56
18. 2.687 p (9)
19. 37.41 4.3
20. 0.098 p 0.55
22. 1.11 0.925
21. 6.025 p 48.2
10. 13.65 p 1.1
14. 8 (3.2)
CLASSZONE.COM
• More Examples
• eTutorial Plus
23. Find the Error Describe and correct the error
9.78
3.4
39 1 2
2934
332.52
in the solution.
24. Balloons You are buying balloons that cost
\$.89 per package to decorate for a school
dance. You have \$14.75 to spend. How many
packages of balloons can you buy?
25. Look for a Pattern Copy and complete the table by multiplying
each number in the leftmost column by the number at the top of each
other column. Describe the pattern.
1
0.1
0.01
0.001
0.0001
87
87
8.7
?
?
?
356
356
?
?
?
?
1200
?
?
?
?
?
Lesson 5.7
Multiplying and Dividing Decimals
253
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26. Lava Flows A lava flow is a stream of molten rock that pours from an
erupting vent. A lava flow travels 15.5 miles down a steep slope in
2.5 hours. Find the average rate at which the flow travels. Write your
Algebra Solve the equation.
a
27. 9 0.9
c
28. 0.16
4.5
29. 1.2x 0.321
30. 8.25y 3.3
Evaluate the expression.
31. 3.4 3 5.1 1.7 4.89
32. 6.2 p (18.77 6.27) 9.1 2
33. Writing Explain how 4.6 divided by 0.23 is related to 460 divided by 23.
Are the quotients the same? Why?
Kilauea Volcano, Hawaii
34. Postal Rates The table shows
rates to mail a first class letter.
How much does it cost to mail
a first class letter that weighs
3.5 ounces?
First ounce or fraction of ounce
\$.37
\$.23
35. Critical Thinking How many decimal places does 1.3 1 have? 1.3 2? 1.3 3?
36. Challenge One micron is equal to 0.001 millimeter. If a bacteria is
4 microns wide, how many times would you have to magnify it for
the bacteria to appear 1 millimeter wide?
Mixed Review
Write the number in standard form. (Lesson 4.8)
37. 6.89 10 9
38. 1.3 10 12
39. 7.405 10 6
Order the numbers from least to greatest. (Lesson 5.5)
3
9
11
40. 2.32, , 2.5, 2 , 2, 10
4
5
9
5
3
41. , 0.46, , , 0.4
20
12
8
Basic Skills Find the quotient.
42. 55 6
43. 127 5
44. 307 29
45. 8607 42
Test-Taking Practice
INTERNET
State Test Practice
CLASSZONE.COM
46. Multiple Choice The quotient 0.57 0.38 is _?_.
A. an integer
B. negative
C. more than 1
D. positive
47. Short Response You have \$75 to spend on party decorations that
cost \$4.89 per bag, including tax. Find how many bags you can buy.
254
Chapter 5
Rational Number Operations
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