Lesson 7
A STORY OF RATIOS
7•2
Lesson 7: Addition and Subtraction of Rational Numbers
Student Outcomes

Students recognize that the rules for adding and subtracting integers apply to rational numbers.

Given a number line, students use arrows to model rational numbers where the length of the arrow is the
absolute value of the rational number and the sign of the rational number is determined by the direction of
the arrow with respect to the number line.

Students locate the sum 𝑝𝑝 + 𝑞𝑞 of two rational numbers on a number line by placing the tail of the arrow for 𝑞𝑞
at 𝑝𝑝 and locating 𝑝𝑝 + 𝑞𝑞 at the head of the arrow. They create an arrow for the difference 𝑝𝑝 − 𝑞𝑞 by first
rewriting the difference as a sum, 𝑝𝑝 + (−𝑞𝑞), and then locating the sum.
Classwork
Exercise 1 (5 minutes): Real-World Connection to Adding and Subtracting Rational Numbers
Students answer the following question independently as the teacher circulates around the room providing guidance
MP.4 and feedback as needed. Students focus on how to represent the answer using both an equation and a number line
diagram.
Exercise 1: Real-World Connection to Adding and Subtracting Rational Numbers
𝟏𝟏
𝟐𝟐
𝟖𝟖
𝟏𝟏
𝟏𝟏𝟏𝟏
𝟏𝟏
𝟏𝟏
𝟔𝟔
𝟏𝟏
𝟒𝟒
𝟏𝟏
𝟎𝟎
𝟐𝟐
𝟏𝟏
𝟏𝟏
𝟏𝟏
𝟏𝟏
𝟏𝟏𝟏𝟏 + �−𝟑𝟑 � = 𝟖𝟖 or 𝟏𝟏𝟏𝟏 − 𝟑𝟑 = 𝟖𝟖
𝟐𝟐
𝟐𝟐
𝟐𝟐
𝟐𝟐
𝟏𝟏
𝟏𝟏
𝟐𝟐𝟐𝟐
equation, and use a number line to model your answer.
𝟏𝟏
Suppose a seventh grader’s birthday is today, and she is 𝟏𝟏𝟏𝟏 years old. How old was she 𝟑𝟑 years ago? Write an
Example 1 (5 minutes): Representing Sums of Rational Numbers on a Number Line
1
2
The teacher leads a whole-group instruction illustrating the sum of 12 + �−3 � on a
MP.6 number line. Elicit student responses to assist in creating the steps. Students record the
steps and diagram.
Example 1: Representing Sums of Rational Numbers on a Number Line
a.
b.
c.
Place the tail of the arrow on 𝟏𝟏𝟏𝟏.
𝟏𝟏
𝟐𝟐
𝟏𝟏
𝟐𝟐
Scaffolding:
Laminate an index card with
the steps for Examples 1 and 2
and the number line diagram
so that students can easily
refer to it.
𝟏𝟏
𝟐𝟐
The length of the arrow is the absolute value of −𝟑𝟑 , �−𝟑𝟑 � = 𝟑𝟑 .
The direction of the arrow is to the left since you are adding a negative number to 𝟏𝟏𝟏𝟏.
Lesson 7:
Addition and Subtraction of Rational Numbers
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Lesson 7
A STORY OF RATIOS
7•2
𝟏𝟏
𝟏𝟏
𝟏𝟏
𝟏𝟏
𝟏𝟏𝟏𝟏 + �−𝟑𝟑 � = 𝟖𝟖 or 𝟏𝟏𝟏𝟏 − 𝟑𝟑 = 𝟖𝟖
𝟐𝟐
𝟐𝟐
𝟐𝟐
𝟐𝟐
𝟏𝟏𝟏𝟏
𝟏𝟏
𝟏𝟏
𝟏𝟏
𝟐𝟐𝟐𝟐
𝟏𝟏
𝟏𝟏
𝟖𝟖
𝟏𝟏
𝟔𝟔
𝟏𝟏
𝟒𝟒
𝟐𝟐
𝟎𝟎
𝟏𝟏
Draw the number line model in the space below.
Exercise 2 (3 minutes)
Exercise 2
𝟏𝟏
𝟐𝟐
𝟏𝟏
𝟏𝟏
�−𝟐𝟐 � + 𝟓𝟓 = 𝟐𝟐
𝟐𝟐
𝟐𝟐
−𝟔𝟔
−𝟒𝟒
−
𝟎𝟎
𝟒𝟒
𝟔𝟔
𝟖𝟖
𝟏𝟏
−𝟖𝟖
𝟐𝟐
−𝟏𝟏
𝟐𝟐
𝟏𝟏
Find the following sum using a number line diagram: −𝟐𝟐 + 𝟓𝟓.
𝟏𝟏
Example 2 (5 minutes): Representing Differences of Rational Numbers on a Number Line
1
4
The teacher leads a whole-group instruction illustrating how to find the difference of 1 − 2 on a number line. Elicit
student responses to assist in creating the steps. Students record the steps and diagram.
Example 2: Representing Differences of Rational Numbers on a Number Line
Have students subdivide their
𝟏𝟏
𝟒𝟒
Find the following difference, and represent it on a number line: 𝟏𝟏 − 𝟐𝟐 .
a.
Rewrite the difference 𝟏𝟏 − 𝟐𝟐
𝟏𝟏
𝟏𝟏
as a sum: 𝟏𝟏 + �−𝟐𝟐 �.
𝟒𝟒
𝟒𝟒
Place the tail of the arrow on 𝟏𝟏.
c.
𝟏𝟏
𝟒𝟒
1
and ’s.
4
𝟏𝟏
𝟒𝟒
1
number line intervals into ’s
Now follow the steps to represent the sum:
b.
Scaffolding:
2
𝟏𝟏
𝟒𝟒
The length of the arrow is the absolute value of −𝟐𝟐 ; �−𝟐𝟐 � = 𝟐𝟐 .
d.
The direction of the arrow is to the left since you are adding a negative number to 𝟏𝟏.
Draw the number line model in the space below.
𝟏𝟏
𝟒𝟒
𝟏𝟏 + �−𝟐𝟐 � = −𝟏𝟏
𝟏𝟏
𝟒𝟒
Lesson 7:
Addition and Subtraction of Rational Numbers
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Lesson 7
A STORY OF RATIOS
7•2
Exercise 3 (3 minutes)
Exercise 3
𝟏𝟏
𝟐𝟐
Find the following difference, and represent it on a number line: −𝟓𝟓 − (−𝟖𝟖).
𝟏𝟏
𝟐𝟐
�−𝟓𝟓 � + 𝟖𝟖 = 𝟐𝟐
𝟏𝟏
𝟐𝟐
Scaffolding:
 Ask students to explain
and justify what they drew
to check for
understanding.
 Ask probing questions
such as “Why does your
arrow go to the right?”
Exercise 4 (10 minutes)
Next, students work independently in Exercise 4 to create a number line model to represent each sum or difference.
After 5–7 minutes, students are selected to share their responses and work with the class.
Exercise 4
Find the following sums and differences using a number line model.
a.
−𝟔𝟔 + 𝟓𝟓
−𝟔𝟔 + 𝟓𝟓
b.
𝟏𝟏
𝟒𝟒
𝟏𝟏
𝟑𝟑
=−
𝟒𝟒
𝟒𝟒
𝟕𝟕 − (−𝟎𝟎. 𝟗𝟗)
𝟕𝟕 + (𝟎𝟎. 𝟗𝟗) = 𝟕𝟕. 𝟗𝟗
c.
𝟏𝟏
𝟐𝟐
𝟐𝟐. 𝟓𝟓 + �− �
𝟐𝟐. 𝟓𝟓 + (−𝟎𝟎. 𝟓𝟓) = 𝟐𝟐
d.
𝟏𝟏
𝟒𝟒
− + 𝟒𝟒
𝟏𝟏
𝟑𝟑
− + 𝟒𝟒 = 𝟑𝟑
𝟒𝟒
𝟒𝟒
e.
𝟏𝟏
𝟐𝟐
− (−𝟑𝟑)
𝟏𝟏
𝟏𝟏
+ 𝟑𝟑 = 𝟑𝟑
𝟐𝟐
𝟐𝟐
Lesson 7:
Addition and Subtraction of Rational Numbers
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Lesson 7
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7•2
Exercise 5 (6 minutes)
Exercise 5
Create an equation and number line diagram to model each answer.
a.
Samantha owes her father $𝟕𝟕. She just got paid $𝟓𝟓. 𝟓𝟓𝟓𝟓 for babysitting. If she gives that money to her dad,
how much will she still owe him?
𝟓𝟓. 𝟓𝟓 units
b.
𝟓𝟓
−𝟕𝟕 + 𝟓𝟓. 𝟓𝟓𝟓𝟓 = −𝟏𝟏. 𝟓𝟓𝟓𝟓. She still owes him $𝟏𝟏. 𝟓𝟓𝟓𝟓.
At the start of a trip, a car’s gas tank contains 𝟏𝟏𝟏𝟏 gallons of gasoline. During the trip, the car consumes 𝟏𝟏𝟏𝟏
gallons of gasoline. How much gasoline is left in the tank?
𝟏𝟏
𝟖𝟖
𝟕𝟕
𝟖𝟖
𝟏𝟏
𝟖𝟖
𝟕𝟕
𝟖𝟖
𝟕𝟕
𝟖𝟖
𝟏𝟏
𝟖𝟖
𝟏𝟏𝟏𝟏 + �−𝟏𝟏𝟏𝟏 � = 𝟏𝟏 or 𝟏𝟏𝟏𝟏 − 𝟏𝟏𝟏𝟏 = 𝟏𝟏 ,There are 𝟏𝟏 gallons of gas left in the tank.
c.
𝟏𝟏
𝟐𝟐
A fish was swimming 𝟑𝟑 feet below the water’s surface at 7:00 a.m. Four hours later, the fish was at a depth
𝟏𝟏
𝟒𝟒
that is 𝟓𝟓 feet below where it was at 7:00 a.m. What rational number represents the position of the fish
with respect to the water’s surface at 11:00 a.m.?
𝟏𝟏
𝟐𝟐
𝟏𝟏
𝟒𝟒
𝟑𝟑
𝟒𝟒
𝟑𝟑
𝟒𝟒
−𝟑𝟑 + �−𝟓𝟓 � = −𝟖𝟖 . The fish is 𝟖𝟖 feet below the water’s surface.
Follow-Up Discussion
For Exercise 5(a), discuss with students how the mathematical answer of −1.50 means Samantha owes her father $1.50
and that we do not say she owes her father −$1.50.
Lesson 7:
Addition and Subtraction of Rational Numbers
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Lesson 7
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7•2
Closing (3 minutes)

What challenges do you face when using the number line model to add non-integer rational numbers?


Answers will vary.
When using a number line to model 8 − (−2.1), how many units do we move from 8 and in what direction?
Where is the tail of the arrow, and where is the head? What does your arrow represent?

First, we would change the expression to an addition expression, 8 + 2.1. The tail of the arrow would
start at 8, the first addend. The arrow would be 2.1 units long and pointing to the right, which would
mean the arrow would end on 10.1. The arrow represents the second addend.
Lesson Summary
The rules for adding and subtracting integers apply to all rational numbers.
The sum of two rational numbers (e.g., −𝟏𝟏 + 𝟒𝟒. 𝟑𝟑) can be found on the number line by placing the tail of an arrow at
−𝟏𝟏 and locating the head of the arrow 𝟒𝟒. 𝟑𝟑 units to the right to arrive at the sum, which is 𝟑𝟑. 𝟑𝟑.
To model the difference of two rational numbers on a number line (e.g., −𝟓𝟓. 𝟕𝟕 − 𝟑𝟑), first rewrite the difference as a
sum, −𝟓𝟓. 𝟕𝟕 + (−𝟑𝟑), and then follow the steps for locating a sum. Place a single arrow with its tail at −𝟓𝟓. 𝟕𝟕 and the
head of the arrow 𝟑𝟑 units to the left to arrive at −𝟖𝟖. 𝟕𝟕.
Exit Ticket (5 minutes)
Lesson 7:
Addition and Subtraction of Rational Numbers
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Lesson 7
A STORY OF RATIOS
Name ___________________________________________________
7•2
Date____________________
Lesson 7: Addition and Subtraction of Rational Numbers
Exit Ticket
At the beginning of the summer, the water level of a pond is 2 feet below its normal level. After an unusually dry
1
3
summer, the water level of the pond dropped another 1 feet.
1.
Use a number line diagram to model the pond’s current water level in relation to its normal water level.
2.
Write an equation to show how far above or below the normal water level the pond is at the end of the summer.
Lesson 7:
Addition and Subtraction of Rational Numbers
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Lesson 7
A STORY OF RATIOS
7•2
Exit Ticket Sample Solutions
At the beginning of the summer, the water level of a pond is 𝟐𝟐 feet below its normal level. After an unusually dry
𝟏𝟏
𝟑𝟑
summer, the water level of the pond dropped another 𝟏𝟏 feet.
1.
Use a number line diagram to model the pond’s current water level in relation to its normal water level.
𝟏𝟏
𝟑𝟑
−𝟒𝟒
2.
−𝟑𝟑
𝟐𝟐
Move 𝟏𝟏 units to the left of −𝟐𝟐. −𝟑𝟑
𝟏𝟏
𝟑𝟑
−
−𝟏𝟏
𝟎𝟎
𝟏𝟏
𝟐𝟐
𝟑𝟑
𝟒𝟒
Write an equation to show how far above or below the normal water level the pond is at the end of the summer.
−𝟐𝟐 − 𝟏𝟏
𝟏𝟏
𝟏𝟏
𝟏𝟏
𝟏𝟏
= −𝟑𝟑 or − 𝟐𝟐 + �−𝟏𝟏 � = −𝟑𝟑
𝟑𝟑
𝟑𝟑
𝟑𝟑
𝟑𝟑
Problem Set Sample Solutions
Represent each of the following problems using both a number line diagram and an equation.
1.
𝟏𝟏
𝟐𝟐
A bird that was perched atop a 𝟏𝟏𝟏𝟏 -foot tree dives down six feet to a branch below. How far above the ground is
the bird’s new location?
𝟏𝟏
𝟐𝟐
𝟏𝟏
𝟐𝟐
𝟏𝟏
𝟐𝟐
𝟏𝟏𝟏𝟏 + (−𝟔𝟔) = 𝟗𝟗 or 𝟏𝟏𝟏𝟏 − 𝟔𝟔 = 𝟗𝟗
𝟏𝟏
𝟐𝟐
𝟏𝟏
𝟐𝟐
The bird is 𝟗𝟗 feet above the ground.
2.
Mariah owed her grandfather $𝟐𝟐. 𝟐𝟐𝟐𝟐 but was recently able to pay him back $𝟏𝟏. 𝟓𝟓𝟓𝟓. How much does Mariah
currently owe her grandfather?
−𝟐𝟐. 𝟐𝟐𝟐𝟐 + 𝟏𝟏. 𝟓𝟓𝟓𝟓 = −𝟎𝟎. 𝟕𝟕𝟕𝟕
Mariah owes her grandfather 𝟕𝟕𝟕𝟕 cents.
Lesson 7:
Addition and Subtraction of Rational Numbers
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Lesson 7
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3.
7•2
Jake is hiking a trail that leads to the top of a canyon. The trail is 𝟒𝟒. 𝟐𝟐 miles long, and Jake plans to stop for lunch
after he completes 𝟏𝟏. 𝟔𝟔 miles. How far from the top of the canyon will Jake be when he stops for lunch?
𝟒𝟒. 𝟐𝟐 − 𝟏𝟏. 𝟔𝟔 = 𝟐𝟐. 𝟔𝟔
Jake will be 𝟐𝟐. 𝟔𝟔 miles from the top of the canyon.
4.
Sonji and her friend Rachel are competing in a running race. When Sonji is 𝟎𝟎. 𝟒𝟒 miles from the finish line, she notices
that her friend Rachel has fallen. If Sonji runs one-tenth of a mile back to help her friend, how far will she be from
the finish line?
−𝟎𝟎. 𝟒𝟒 + (−𝟎𝟎. 𝟏𝟏) = −𝟎𝟎. 𝟓𝟓 or −𝟎𝟎. 𝟒𝟒 − 𝟎𝟎. 𝟏𝟏 = −𝟎𝟎. 𝟓𝟓
Sonji will be 𝟎𝟎. 𝟓𝟓 miles from the finish line.
5.
Mr. Henderson did not realize his checking account had a balance of $𝟐𝟐𝟐𝟐𝟐𝟐 when he used his debit card for a
$𝟑𝟑𝟑𝟑𝟑𝟑. 𝟐𝟐𝟐𝟐 purchase. What is his checking account balance after the purchase?
𝟐𝟐𝟐𝟐𝟐𝟐 + (−𝟑𝟑𝟑𝟑𝟑𝟑. 𝟐𝟐𝟐𝟐) = −𝟏𝟏𝟏𝟏𝟏𝟏. 𝟐𝟐𝟐𝟐 or 𝟐𝟐𝟐𝟐𝟐𝟐 − 𝟑𝟑𝟑𝟑𝟑𝟑. 𝟐𝟐𝟐𝟐 = −𝟏𝟏𝟏𝟏𝟏𝟏. 𝟐𝟐𝟐𝟐
Mr. Henderson’s checking account balance will be −$𝟏𝟏𝟏𝟏𝟏𝟏. 𝟐𝟐𝟐𝟐.
6.
If the temperature is −𝟑𝟑°𝐅𝐅 at 10:00 p.m., and the temperature falls four degrees overnight, what is the resulting
temperature?
−𝟑𝟑 − 𝟒𝟒 = −𝟑𝟑 + (−𝟒𝟒) = −𝟕𝟕
The resulting temperature is −𝟕𝟕℉.
Lesson 7:
Addition and Subtraction of Rational Numbers
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