Foundations
5 Multiply and Divide Integers
Learning Objectives
By the end of this section, you will be able to:
 Multiply integers
 Divide integers
 Simplify expressions with integers
 Evaluate variable expressions with integers
 Translate English phrases to algebraic expressions
 Use integers in applications
A more thorough introduction to the topics covered in this section can be found in the Prealgebra chapter, Integers.
Multiply Integers
Since multiplication is mathematical shorthand for repeated addition, our model can easily be applied to show multiplication of integers. Let’s look at this concrete model to see what patterns we notice. We will use the same examples that we used for addition and subtraction. Here, we will use the model just to help us discover the pattern.
We remember that means add a, b times. Here, we are using the model just to help us discover the pattern.
The next two examples are more interesting.
What does it mean to multiply 5 by It means subtract 5, 3 times. Looking at subtraction as “taking away,” it means to take away 5, 3 times. But there is nothing to take away, so we start by adding neutral pairs on the workspace. Then we take away 5 three times.
In summary:
Notice that for multiplication of two signed numbers, when the:
 signs are the same, the product is positive.
 signs are different, the product is negative.
We’ll put this all together in the chart below.
For multiplication of two signed numbers:
Same signs  Product  Example 

Two positives Two negatives 
Positive Positive 
Different signs  Product  Example 

Positive · negative Negative · positive 
Negative Negative 
Multiply: ⓐ ⓑ ⓒ ⓓ
ⓐ Multiply, noting that the signs are different so the product is negative. 

ⓑ Multiply, noting that the signs are the same so the product is positive. 

ⓒ Multiply, with different signs. 

ⓓ Multiply, with same signs. 
Multiply: ⓐ ⓑ ⓒ ⓓ
ⓐⓑ 28 ⓒ ⓓ 60
Multiply: ⓐ ⓑ ⓒ ⓓ
ⓐⓑ 54 ⓒ ⓓ 39
When we multiply a number by 1, the result is the same number. What happens when we multiply a number by Let’s multiply a positive number and then a negative number by to see what we get.
Each time we multiply a number by we get its opposite!
Multiplying a number by gives its opposite.
Multiply: ⓐ ⓑ
ⓐ Multiply, noting that the signs are different so the product is negative. 

ⓑ Multiply, noting that the signs are the same so the product is positive. 
Multiply: ⓐ ⓑ
ⓐⓑ 17
Multiply: ⓐ ⓑ
ⓐⓑ 16
Divide Integers
What about division? Division is the inverse operation of multiplication. So, because In words, this expression says that 15 can be divided into three groups of five each because adding five three times gives 15. Look at some examples of multiplying integers, to figure out the rules for dividing integers.
Division follows the same rules as multiplication!
For division of two signed numbers, when the:
 signs are the same, the quotient is positive.
 signs are different, the quotient is negative.
And remember that we can always check the answer of a division problem by multiplying.
For multiplication and division of two signed numbers:
 If the signs are the same, the result is positive.
 If the signs are different, the result is negative.
Same signs  Result 

Two positives Two negatives 
Positive Positive 
If the signs are the same, the result is positive. 
Different signs  Result 

Positive and negative Negative and positive 
Negative Negative 
If the signs are different, the result is negative. 
Divide: ⓐ ⓑ
ⓐ Divide. With different signs, the quotient is negative. 

ⓑ Divide. With signs that are the same, the quotient is positive. 
Divide: ⓐ ⓑ
ⓐⓑ 39
Divide: ⓐ ⓑ
ⓐⓑ 23
Simplify Expressions with Integers
What happens when there are more than two numbers in an expression? The order of operations still applies when negatives are included. Remember My Dear Aunt Sally?
Let’s try some examples. We’ll simplify expressions that use all four operations with integers—addition, subtraction, multiplication, and division. Remember to follow the order of operations.
Simplify:
Multiply first.  
Add.  
Subtract. 
Simplify:
Simplify:
Simplify: ⓐ ⓑ
ⓐ Write in expanded form. Multiply. Multiply. Multiply. 

ⓑ Write in expanded form. We are asked to find the opposite of Multiply. Multiply. Multiply. 
Notice the difference in parts ⓐ and ⓑ. In part ⓐ, the exponent means to raise what is in the parentheses, the to the power. In part ⓑ, the exponent means to raise just the 2 to the power and then take the opposite.
Simplify: ⓐ ⓑ
ⓐ 81 ⓑ
Simplify: ⓐ ⓑ
ⓐ 49 ⓑ
The next example reminds us to simplify inside parentheses first.
Simplify:
Subtract in parentheses first.  
Multiply.  
Subtract. 
Simplify:
29
Simplify:
52
Simplify:
Exponents first.  
Multiply.  
Divide. 
Simplify:
4
Simplify:
9
Simplify:
Multiply and divide left to right, so divide first.  
Multiply.  
Add. 
Simplify:
21
Simplify:
6
Evaluate Variable Expressions with Integers
Remember that to evaluate an expression means to substitute a number for the variable in the expression. Now we can use negative numbers as well as positive numbers.
When evaluate: ⓐ ⓑ
ⓐ
Simplify.  −4 
ⓑ
Simplify.  
Add.  6 
When evaluate ⓐ ⓑ
ⓐⓑ 10
When evaluate ⓐ ⓑ
ⓐⓑ 17
Evaluate when and
Add inside parenthesis.  (6)^{2} 
Simplify.  36 
Evaluate when and
196
Evaluate when and
8
Evaluate when ⓐ and ⓑ
ⓐ
Subtract.  8 
ⓑ
Subtract.  32 
Evaluate: when ⓐ and ⓑ
ⓐⓑ 36
Evaluate: when ⓐ and ⓑ
ⓐⓑ 9
Evaluate: when
Substitute Use parentheses to show multiplication.
Substitute.  
Evaluate exponents.  
Multiply.  
Add.  52 
Evaluate: when
39
Evaluate: when
13
Translate Phrases to Expressions with Integers
Our earlier work translating English to algebra also applies to phrases that include both positive and negative numbers.
Translate and simplify: the sum of 8 and increased by 3.
the sum of 8 and increased by 3.  
Translate.  
Simplify. Be careful not to confuse the brackets with an absolute value sign.  
Add. 
Translate and simplify the sum of 9 and increased by 4.
Translate and simplify the sum of and increased by 7.
When we first introduced the operation symbols, we saw that the expression may be read in several ways. They are listed in the chart below.
minus the difference of and subtracted from less than 
Be careful to get a and b in the right order!
Translate and then simplify ⓐ the difference of 13 and ⓑ subtract 24 from
ⓐ Translate. Simplify. 

ⓑ Translate. Remember, “subtract from means . Simplify. 
Translate and simplify ⓐ the difference of 14 and ⓑ subtract 21 from
ⓐⓑ
Translate and simplify ⓐ the difference of 11 and ⓑ subtract 18 from
ⓐⓑ
Once again, our prior work translating English to algebra transfers to phrases that include both multiplying and dividing integers. Remember that the key word for multiplication is “product” and for division is “quotient.”
Translate to an algebraic expression and simplify if possible: the product of and 14.
Translate.  
Simplify. 
Translate to an algebraic expression and simplify if possible: the product of and 12.
Translate to an algebraic expression and simplify if possible: the product of 8 and
Translate to an algebraic expression and simplify if possible: the quotient of and
Translate.  
Simplify. 
Translate to an algebraic expression and simplify if possible: the quotient of and
Translate to an algebraic expression and simplify if possible: the quotient of and
Use Integers in Applications
We’ll outline a plan to solve applications. It’s hard to find something if we don’t know what we’re looking for or what to call it! So when we solve an application, we first need to determine what the problem is asking us to find. Then we’ll write a phrase that gives the information to find it. We’ll translate the phrase into an expression and then simplify the expression to get the answer. Finally, we summarize the answer in a sentence to make sure it makes sense.
The temperature in Urbana, Illinois one morning was 11 degrees. By midafternoon, the temperature had dropped to degrees. What was the difference of the morning and afternoon temperatures?
The temperature in Anchorage, Alaska one morning was 15 degrees. By midafternoon the temperature had dropped to 30 degrees below zero. What was the difference in the morning and afternoon temperatures?
The difference in temperatures was 45 degrees.
The temperature in Denver was degrees at lunchtime. By sunset the temperature had dropped to degrees. What was the difference in the lunchtime and sunset temperatures?
The difference in temperatures was 9 degrees.
 Read the problem. Make sure all the words and ideas are understood
 Identify what we are asked to find.
 Write a phrase that gives the information to find it.
 Translate the phrase to an expression.
 Simplify the expression.
 Answer the question with a complete sentence.
The Mustangs football team received three penalties in the third quarter. Each penalty gave them a loss of fifteen yards. What is the number of yards lost?
Step 1. Read the problem. Make sure all the words and ideas are understood.  
Step 2. Identify what we are asked to find.  the number of yards lost 
Step 3. Write a phrase that gives the information to find it.  three times a 15yard penalty 
Step 4. Translate the phrase to an expression.  
Step 5. Simplify the expression.  
Step 6. Answer the question with a complete sentence.  The team lost 45 yards. 
The Bears played poorly and had seven penalties in the game. Each penalty resulted in a loss of 15 yards. What is the number of yards lost due to penalties?
The Bears lost 105 yards.
Bill uses the ATM on campus because it is convenient. However, each time he uses it he is charged a ?2 fee. Last month he used the ATM eight times. How much was his total fee for using the ATM?
A ?16 fee was deducted from his checking account.
Key Concepts
 Multiplication and Division of Two Signed Numbers
 Same signs—Product is positive
 Different signs—Product is negative
 Strategy for Applications
 Identify what you are asked to find.
 Write a phrase that gives the information to find it.
 Translate the phrase to an expression.
 Simplify the expression.
 Answer the question with a complete sentence.
Practice Makes Perfect
Multiply Integers
In the following exercises, multiply.
14
Divide Integers
In the following exercises, divide.
13
Simplify Expressions with Integers
In the following exercises, simplify each expression.
64
90
9
41
5
Evaluate Variable Expressions with Integers
In the following exercises, evaluate each expression.
when
ⓐ
ⓑ
ⓐⓑ 16
when
ⓐ
ⓑ
ⓐ when
ⓑ when
ⓐⓑ 10
ⓐ when
ⓑ when
when
when
when
when
when
121
when
when
ⓐ
ⓑ
ⓐ 1 ⓑ 33
when
ⓐ
ⓑ
when
ⓐ
ⓑ
ⓐⓑ 25
when
ⓐ
ⓑ
when
21
when
when
when
Translate English Phrases to Algebraic Expressions
In the following exercises, translate to an algebraic expression and simplify if possible.
the sum of 3 and increased by 7
the sum of and increased by 23
the difference of 10 and
subtract 11 from
the difference of and
subtract from
the product of
the product of
the quotient of and
the quotient of and
the quotient of and the sum of a and b
the quotient of and the sum of m and n
the product of and the difference of
the product of and the difference of
Use Integers in Applications
In the following exercises, solve.
Temperature On January the high temperature in Anaheim, California, was That same day, the high temperature in Embarrass, Minnesota was What was the difference between the temperature in Anaheim and the temperature in Embarrass?
Temperature On January the high temperature in Palm Springs, California, was and the high temperature in Whitefield, New Hampshire was What was the difference between the temperature in Palm Springs and the temperature in Whitefield?
Football At the first down, the Chargers had the ball on their 25 yard line. On the next three downs, they lost 6 yards, gained 10 yards, and lost 8 yards. What was the yard line at the end of the fourth down?
21
Football At the first down, the Steelers had the ball on their 30 yard line. On the next three downs, they gained 9 yards, lost 14 yards, and lost 2 yards. What was the yard line at the end of the fourth down?
Checking Account Mayra has ?124 in her checking account. She writes a check for ?152. What is the new balance in her checking account?
Checking Account Selina has ?165 in her checking account. She writes a check for ?207. What is the new balance in her checking account?
Checking Account Diontre has a balance of in his checking account. He deposits ?225 to the account. What is the new balance?
?187
Checking Account Reymonte has a balance of in his checking account. He deposits ?281 to the account. What is the new balance?
Everyday Math
Stock market Javier owns 300 shares of stock in one company. On Tuesday, the stock price dropped ?12 per share. What was the total effect on Javier’s portfolio?
Weight loss In the first week of a diet program, eight women lost an average of 3 pounds each. What was the total weight change for the eight women?
Writing Exercises
In your own words, state the rules for multiplying integers.
Answers may vary
In your own words, state the rules for dividing integers.
Why is
Answers may vary
Why is
Self Check
ⓐ After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.
ⓑ On a scale of 1–10, how would you rate your mastery of this section in light of your responses on the checklist? How can you improve this?