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```ALGEBRA
Name
Lesson 7.9
Essential Question How can you add fractions with like denominators
connect The Associative and Commutative Properties of Addition
use mental math to combine fractions that have a sum of 1.
•
The Commutative Property of Addition states that when
the order of two addends is changed, the sum is the
same. For example, 4 + 5 = 5 + 4.
•
The Associative Property of Addition states that when the
grouping of addends is changed, the sum is the same. For
example, (5 + 8) + 4 = 5 + (8 + 4).
Numbers and Operations—
Fractions—4.NF.B.3c
MATHEMATICAL PRACTICES
MP2, MP7
Unlock
Unlock the
the Problem
Problem
The map shows four lighthouses in the Florida Keys
and their distances apart in miles. The Dry Tortugas
Lighthouse is the farthest west, and the Alligator
Reef Lighthouse is the farthest east.
70 150
43 160
34 150
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What is the distance from the Dry Tortugas Lighthouse
to the Alligator Reef Lighthouse, traveling between the
four lighthouses?
Use the properties to order and group.
5 + 43 ___
6 + 34 ___
5
10
10
10
6 + 34 ___
5 + 43 ___
5 =_+_+_
70 ___
10
10
10
Use the Commutative
Property to order the
with a sum of 1 are together.
= (_ + _) + _
Use the Associative Property
= (_) + _
mixed number.
=_
Write the sum.
So, the distance from the Dry Tortugas Lighthouse to the
Alligator Reef Lighthouse, traveling between the four lighthouses,
is _ miles.
Chapter 7
435
Try This! Use the properties and mental math to solve. Show each
step, and name the property used.
1 1_3 + (2 + 3 2_3 )
Share
hhow
Share and
and Show
Sh
MATH
M
BOARD
B
1. Complete. Name the property used.
(3104 + 5102 ) + 106 = (5102 + 3104 ) + _
= 5 2 + (3 4 + _)
10
10
____
____
____
____
____
____
_____
____
_____
2 +
= 5 ____
_
Math
Talk
10
=_
MATHEMATICAL PRACTICES 2
Reason Abstractly
Describe how you could
use the properties to find
the sum 1 1_3 + 2 5_8 +1 2_3 .
Use the properties and mental math to find the sum.
2.
(278 + 328) + 118
3.
__ + 1 + 3
__
12
4.
5.
__ + 11
__) + 2 3
__
(11
6.
__ + 1 2
__) + 3 5
__
(12 4
9
8
3
7. ____ + (1____ + ____)
436
4
__
4
__
4
5
(
9
5)
9
9
__ + (5 5
__ + 4 3
__)
53
6
12
6
12
6
12
© Houghton Mifflin Harcourt Publishing Company
__
Name
On
On Your
Own
Use the properties and mental math to find the sum.
8.
11.
(451_3 + 61_3) + 382_3
(
)
(
DEEPER
Pablo is training for a marathon.
4
_
He ran 5 8 miles on Friday, 6 5_8 miles on
Saturday, and 7 4_8 miles on Sunday. How
many miles did he run on all three days?
12.
)
5 + 10 + 11__
5
10. 3 __
10
10
9. _1 + 103 1_ + 12
2
2
DEEPER
At lunchtime, Dale’s Diner served
pots of vegetable soup, 3 5_6 pots
of chicken soup, and 4 3_6 pots of tomato soup.
How many pots of soup were served in all?
a total of 2 _26
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OqnakdlRnkuhmf¤@ookhb`shnmr
Use the expressions in the box for 13–14.
13. Which property of addition would you use to
(
)
A _1 + 7_8 + 4_
8
8
regroup the addends in Expression A?
B _1 + 2
2
14.
SMARTER
Which two expressions have the
(
)
C _37 + 1_2 + 4_7
same value?
D _1 + 4_ + 2_
© Houghton Mifflin Harcourt Publishing Company
3
15.
SMARTER
3
3
Match the equation with the property used.
6
6
3
6
6
3
__
__
__
__
__
__
12 + ( 12 + 12 ) = ( 1 2 + 12 ) + 12
3 2_5 + (5 4_5 + 2 1_5 ) = 3 2_5 + (2 1_5 + 5 4_5 )
(4 1_6 + 3 5_6 ) + 2 2_6 = (3 5_6 + 4 1_6 ) + 2 2_6
(1 1_8 + 5_8 ) + 3 3_8 = 1 1_8 + ( 5_8 + 3 3_8 )
•
•
•
•
•
•
Commutative Property
Associative Property
Chapter 7 • Lesson 9 437
MATHEMATICAL PRACTICES
ANALYZEt-00,'034536\$563&t13&\$*4*0/
Pose a Problem
DEEPER
16.
Costumes are being made for the
high school musical. The table at the right
shows the amount of fabric needed for the
costumes of the male and female leads. Alice
uses the expression 7 3_8 + 1 5_8 + 2 4_8 to find the total
amount of fabric needed for the costume of the
To find the value of the expression using mental
math, Alice used the properties of addition.
Costume Fabric (in Yards)
Costume
Costume
Silk
7 3_8
1 2_8
Felt
1 5_8
2 3_8
Cotton
2 4_8
5 6_8
Material
__ + 15
__ + 24
__ = 73
__ + 15
__ + 24
__
73
8
8
8
(
)
8
8
8
3_ and 5
_ to the sum of 8 to get 9. She added 24
_
8
8
8
4
to 9, so her answer was 11_.
8
So, the amount of fabric needed for the costume of the female lead
actor is 114_ yards.
8
•
MATHEMATICAL
PRACTICE
Identify Relationships Explain how using the properties of
addition makes both problems easier to solve.
438
7
© Houghton Mifflin Harcourt Publishing Company
Write a new problem using the information for the costume for
Pose a Problem
Practice and Homework
Name
Lesson 7.9
COMMON CORE STANDARD—4.NF.B.3c
Build fractions from unit fractions by applying
and extending previous understandings of
operations on whole numbers.
Use the properties and mental math to find the sum.
2 1 1 1__
__ 1 2 __
1. 5 1
3
(
3
)
3
__ 1 3 5
__ 1 2 7
__
2. 10 1
8
(
8
)
8
__ 1 3 2
__ 1 5 4
__
3. 8 1
5
(
)
5
5
1 1 (4)
5_
3
9 1_3
_
_
2 1 5 1__
__ 1 4 __
4. 6 3
4
(
4
)
4
_
5.
_
(6 3__6 1 10 4__6) 1 9 __62
_
6.
(6 2__5 1 1 4__5) 1 3 __51
_
Problem
Problem Solving
Solving
7. Nate’s classroom has three tables of different
© Houghton Mifflin Harcourt Publishing Company
lengths. One has a length of 4 1_2 feet, another
has a length of 4 feet, and a third has a length
of 2 1_2 feet. What is the length of all three
tables when pushed end to end?
9.
8. Mr. Warren uses 2 1_4 bags of mulch for his
garden and another 4 1_4 bags for his front
yard. He also uses 3_4 bag around a fountain.
How many total bags of mulch does
Mr. Warren use?
Math Describe how the Commutative and Associative
WRITE
Chapter 7
439
Lesson Check (4.NF.B.3c)
1. A carpenter cut a board into three pieces.
_5
6
One piece was 2 feet long. The second
piece was 3 1_6 feet long. The third piece was
1 5_6 feet long. How long was the board?
2. Harry works at an apple orchard. He
picked 45 7_8 pounds of apples on Monday.
He picked 42 3_8 pounds of apples on
Wednesday. He picked 54 1_8 pounds of
apples on Friday. How many pounds of
apples did Harry pick those three days?
Spiral Review (4.OA.B.4, 4.NBT.B.5, 4.NBT.B.6, 4.NF.B.3c)
3. There were 6 oranges in the refrigerator.
_2
3
Joey and his friends ate 3 oranges.
How many oranges were left?
4. Darlene was asked to identify which of the
following numbers is prime:
2, 12, 21, 39
Which number should she choose?
assembly into equal rows. Write one way
the chairs could be arranged. Include the
number of rows and the number of chairs in
each row.
6. Nic bought 28 folding chairs for \$16 each.
How much money did Nic spend on chairs?
FOR MORE PRACTICE
GO TO THE
440
Personal Math Trainer
© Houghton Mifflin Harcourt Publishing Company
5. A teacher has 100 chairs to arrange for an
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