Learning Objectives
By the end of this section, you will be able to:
- Identify multiples of numbers
- Use common divisibility tests
- Find all the factors of a number
- Identify prime and composite numbers
Before you get started, take this readiness quiz.
Which of the following numbers are counting numbers (natural numbers)?
$0,4,215$
If you missed this problem, review Example 1.1.
Find the sum of $3,5,$ and $7.$
If you missed the problem, review Example 2.1.
Identify Multiples of Numbers
Annie is counting the shoes in her closet. The shoes are matched in pairs, so she doesn’t have to count each one. She counts by twos: $2,4,6,8,10,12.$ She has $12$ shoes in her closet.
The numbers $2,4,6,8,10,12$ are called multiples of $2.$ Multiples of $2$ can be written as the product of a counting number and $2.$ The first six multiples of $2$ are given below.
A multiple of a number is the product of the number and a counting number. So a multiple of $3$ would be the product of a counting number and $3.$ Below are the first six multiples of $3.$
We can find the multiples of any number by continuing this process. Table 2.8 shows the multiples of $2$ through $9$ for the first twelve counting numbers.
Counting Number | $1$ | $2$ | $3$ | $4$ | $5$ | $6$ | $7$ | $8$ | $9$ | $10$ | $11$ | $12$ |
---|---|---|---|---|---|---|---|---|---|---|---|---|
$\text{Multiples of}\phantom{\rule{0.2em}{0ex}}2$ | $2$ | $4$ | $6$ | $8$ | $10$ | $12$ | $14$ | $16$ | $18$ | $20$ | $22$ | $24$ |
$\text{Multiples of}\phantom{\rule{0.2em}{0ex}}3$ | $3$ | $6$ | $9$ | $12$ | $15$ | $18$ | $21$ | $24$ | $27$ | $30$ | $33$ | $36$ |
$\text{Multiples of}\phantom{\rule{0.2em}{0ex}}4$ | $4$ | $8$ | $12$ | $16$ | $20$ | $24$ | $28$ | $32$ | $36$ | $40$ | $44$ | $48$ |
$\text{Multiples of}\phantom{\rule{0.2em}{0ex}}5$ | $5$ | $10$ | $15$ | $20$ | $25$ | $30$ | $35$ | $40$ | $45$ | $50$ | $55$ | $60$ |
$\text{Multiples of}\phantom{\rule{0.2em}{0ex}}6$ | $6$ | $12$ | $18$ | $24$ | $30$ | $36$ | $42$ | $48$ | $54$ | $60$ | $66$ | $72$ |
$\text{Multiples of}\phantom{\rule{0.2em}{0ex}}7$ | $7$ | $14$ | $21$ | $28$ | $35$ | $42$ | $49$ | $56$ | $63$ | $70$ | $77$ | $84$ |
$\text{Multiples of}\phantom{\rule{0.2em}{0ex}}8$ | $8$ | $16$ | $24$ | $32$ | $40$ | $48$ | $56$ | $64$ | $72$ | $80$ | $88$ | $96$ |
$\text{Multiples of}\phantom{\rule{0.2em}{0ex}}9$ | $9$ | $18$ | $27$ | $36$ | $45$ | $54$ | $63$ | $72$ | $81$ | $90$ | $99$ | $108$ |
Multiple of a Number
A number is a multiple of $n$ if it is the product of a counting number and $n.$
Recognizing the patterns for multiples of $2,5,10,\text{and}\phantom{\rule{0.2em}{0ex}}3$ will be helpful to you as you continue in this course.
Manipulative Mathematics
Figure 2.6 shows the counting numbers from $1$ to $50.$ Multiples of $2$ are highlighted. Do you notice a pattern?
The last digit of each highlighted number in Figure 2.6 is either $0,2,4,6,\text{or}\phantom{\rule{0.2em}{0ex}}8.$ This is true for the product of $2$ and any counting number. So, to tell if any number is a multiple of $2$ look at the last digit. If it is $0,2,4,6,\phantom{\rule{0.2em}{0ex}}\text{or}\phantom{\rule{0.2em}{0ex}}8,$ then the number is a multiple of $2.$
Example 2.40
Determine whether each of the following is a multiple of $2\text{:}$
- ⓐ$\phantom{\rule{0.2em}{0ex}}489\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}\mathrm{3,714}$
Determine whether each number is a multiple of $2\text{:}$
- ⓐ $\phantom{\rule{0.2em}{0ex}}678\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}\mathrm{21,493}$
Determine whether each number is a multiple of $2\text{:}$
- ⓐ $\phantom{\rule{0.2em}{0ex}}979\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}\mathrm{17,780}$
Now let’s look at multiples of $5.$ Figure 2.7 highlights all of the multiples of $5$ between $1$ and $50.$ What do you notice about the multiples of $5?$
All multiples of $5$ end with either $5$ or $0.$ Just like we identify multiples of $2$ by looking at the last digit, we can identify multiples of $5$ by looking at the last digit.
Example 2.41
Determine whether each of the following is a multiple of $5\text{:}\phantom{\rule{0.2em}{0ex}}$
- ⓐ $\phantom{\rule{0.2em}{0ex}}579\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}880$
Determine whether each number is a multiple of $5.$
- ⓐ $\phantom{\rule{0.2em}{0ex}}675\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}\mathrm{1,578}$
Determine whether each number is a multiple of $5.$
- ⓐ $\phantom{\rule{0.2em}{0ex}}421\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}\mathrm{2,690}$
Figure 2.8 highlights the multiples of $10$ between $1$ and $50.$ All multiples of $10$ all end with a zero.
Example 2.42
Determine whether each of the following is a multiple of $10\text{:}\phantom{\rule{0.2em}{0ex}}$
- ⓐ $\phantom{\rule{0.2em}{0ex}}425\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}350$
Determine whether each number is a multiple of $10\text{:}$
- ⓐ $\phantom{\rule{0.2em}{0ex}}179\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}\mathrm{3,540}$
Determine whether each number is a multiple of $10\text{:}$
- ⓐ $\phantom{\rule{0.2em}{0ex}}110\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}\mathrm{7,595}$
Figure 2.9 highlights multiples of $3.$ The pattern for multiples of $3$ is not as obvious as the patterns for multiples of $2,5,\text{and}\phantom{\rule{0.2em}{0ex}}10.$
Unlike the other patterns we’ve examined so far, this pattern does not involve the last digit. The pattern for multiples of $3$ is based on the sum of the digits. If the sum of the digits of a number is a multiple of $3,$ then the number itself is a multiple of $3.$ See Table 2.9.
$\mathbf{\text{Multiple of 3}}$ | $3$ | $6$ | $9$ | $12$ | $15$ | $18$ | $21$ | $24$ |
$\mathbf{\text{Sum of digits}}$ | $3$ | $6$ | $9$ | $\begin{array}{c}\hfill 1+2\hfill \\ \hfill 3\hfill \end{array}$ | $\begin{array}{c}\hfill 1+5\hfill \\ \hfill 6\hfill \end{array}$ | $\begin{array}{c}\hfill 1+8\hfill \\ \hfill 9\hfill \end{array}$ | $\begin{array}{c}\hfill 2+1\hfill \\ \hfill 3\hfill \end{array}$ | $\begin{array}{c}\hfill 2+4\hfill \\ \hfill 6\hfill \end{array}$ |
Consider the number $42.$ The digits are $4$ and $2,$ and their sum is $4+2=6.$ Since $6$ is a multiple of $3,$ we know that $42$ is also a multiple of $3.$
Example 2.43
Determine whether each of the given numbers is a multiple of $3\text{:}\phantom{\rule{0.2em}{0ex}}$
- ⓐ $\phantom{\rule{0.2em}{0ex}}645\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}\mathrm{10,519}$
Determine whether each number is a multiple of $3\text{:}$
- ⓐ $\phantom{\rule{0.2em}{0ex}}954\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}\mathrm{3,742}$
Determine whether each number is a multiple of $3\text{:}$
- ⓐ $\phantom{\rule{0.2em}{0ex}}643\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}\mathrm{8,379}$
Look back at the charts where you highlighted the multiples of $2,$ of $5,$ and of $10.$ Notice that the multiples of $10$ are the numbers that are multiples of both $2$ and $5.$ That is because $10=2\cdot 5.$ Likewise, since $6=2\cdot 3,$ the multiples of $6$ are the numbers that are multiples of both $2$ and $3.$
Use Common Divisibility Tests
Another way to say that $375$ is a multiple of $5$ is to say that $375$ is divisible by $5.$ In fact, $375\xf75$ is $75,$ so $375$ is $5\cdot 75.$ Notice in Example 2.43 that $\mathrm{10,519}$ is not a multiple $3.$ When we divided $\mathrm{10,519}$ by $3$ we did not get a counting number, so $\mathrm{10,519}$ is not divisible by $3.$
Divisibility
If a number $m$ is a multiple of $n,$ then we say that $m$ is divisible by $n.$
Since multiplication and division are inverse operations, the patterns of multiples that we found can be used as divisibility tests. Table 2.10 summarizes divisibility tests for some of the counting numbers between one and ten.
Divisibility Tests | |
---|---|
A number is divisible by | |
$2$ | if the last digit is $0,\phantom{\rule{0.2em}{0ex}}2,\phantom{\rule{0.2em}{0ex}}4,\phantom{\rule{0.2em}{0ex}}6,\phantom{\rule{0.2em}{0ex}}\text{or}\phantom{\rule{0.2em}{0ex}}8$ |
$3$ | if the sum of the digits is divisible by $3$ |
$5$ | if the last digit is $5$ or $0$ |
$6$ | if divisible by both $2$ and $3$ |
$10$ | if the last digit is $0$ |
Example 2.44
Determine whether $\mathrm{1,290}$ is divisible by $2,3,5,\text{and}\phantom{\rule{0.2em}{0ex}}10.$
Determine whether the given number is divisible by $2,3,5,\text{and}\phantom{\rule{0.2em}{0ex}}10.$
$6240$
Determine whether the given number is divisible by $2,3,5,\text{and}\phantom{\rule{0.2em}{0ex}}10.$
$7248$
Example 2.45
Determine whether $\mathrm{5,625}$ is divisible by $2,3,5,\text{and}\phantom{\rule{0.2em}{0ex}}10.$
Determine whether the given number is divisible $\text{by}\phantom{\rule{0.2em}{0ex}}2,3,5,\text{and}\phantom{\rule{0.2em}{0ex}}10.$
$4962$
Determine whether the given number is divisible $\text{by}\phantom{\rule{0.2em}{0ex}}2,3,5,\text{and}\phantom{\rule{0.2em}{0ex}}10.$
$3765$
Find All the Factors of a Number
There are often several ways to talk about the same idea. So far, we’ve seen that if $m$ is a multiple of $n,$ we can say that $m$ is divisible by $n.$ We know that $72$ is the product of $8$ and $9,$ so we can say $72$ is a multiple of $8$ and $72$ is a multiple of $9.$ We can also say $72$ is divisible by $8$ and by $9.$ Another way to talk about this is to say that $8$ and $9$ are factors of $72.$ When we write $72=8\cdot 9$ we can say that we have factored $72.$
Factors
In the expression $a\cdot b$, both a and b are called factors. If $a\cdot b=m,$ and both a and b are integers, then $a\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}b$ are factors of $m,$ and $m$ is the product of $a\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}b.$
In algebra, it can be useful to determine all of the factors of a number. This is called factoring a number, and it can help us solve many kinds of problems.
Manipulative Mathematics
For example, suppose a choreographer is planning a dance for a ballet recital. There are $24$ dancers, and for a certain scene, the choreographer wants to arrange the dancers in groups of equal sizes on stage.
In how many ways can the dancers be put into groups of equal size? Answering this question is the same as identifying the factors of $24.$ Table 2.13 summarizes the different ways that the choreographer can arrange the dancers.
Number of Groups | Dancers per Group | Total Dancers |
---|---|---|
$1$ | $24$ | $1\cdot 24=24$ |
$2$ | $12$ | $2\cdot 12=24$ |
$3$ | $8$ | $3\cdot 8=24$ |
$4$ | $6$ | $4\cdot 6=24$ |
$6$ | $4$ | $6\cdot 4=24$ |
$8$ | $3$ | $8\cdot 3=24$ |
$12$ | $2$ | $12\cdot 2=24$ |
$24$ | $1$ | $24\cdot 1=24$ |
What patterns do you see in Table 2.13? Did you notice that the number of groups times the number of dancers per group is always $24?$ This makes sense, since there are always $24$ dancers.
You may notice another pattern if you look carefully at the first two columns. These two columns contain the exact same set of numbers—but in reverse order. They are mirrors of one another, and in fact, both columns list all of the factors of $24,$ which are:
We can find all the factors of any counting number by systematically dividing the number by each counting number, starting with $1.$ If the quotient is also a counting number, then the divisor and the quotient are factors of the number. We can stop when the quotient becomes smaller than the divisor.
How To
Find all the factors of a counting number.
- Step 1. Divide the number by each of the counting numbers, in order, until the quotient is smaller than the divisor.
- If the quotient is a counting number, the divisor and quotient are a pair of factors.
- If the quotient is not a counting number, the divisor is not a factor.
- Step 2. List all the factor pairs.
- Step 3. Write all the factors in order from smallest to largest.
Example 2.46
Find all the factors of $72.$
Find all the factors of the given number:
$96$
Find all the factors of the given number:
$80$
Identify Prime and Composite Numbers
Some numbers, like $72,$ have many factors. Other numbers, such as $7,$ have only two factors: $1$ and the number. A number with only two factors is called a prime number. A number with more than two factors is called a composite number. The number $1$ is neither prime nor composite. It has only one factor, itself.
Prime Numbers and Composite Numbers
A prime number is a counting number greater than $1$ whose only factors are $1$ and itself.
A composite number is a counting number that is not prime.
Figure 2.10 lists the counting numbers from $2$ through $20$ along with their factors. The highlighted numbers are prime, since each has only two factors.
The prime numbers less than $20$ are $2,3,5,7,11,13,17,\text{and}\phantom{\rule{0.2em}{0ex}}19.$ There are many larger prime numbers too. In order to determine whether a number is prime or composite, we need to see if the number has any factors other than $1$ and itself. To do this, we can test each of the smaller prime numbers in order to see if it is a factor of the number. If none of the prime numbers are factors, then that number is also prime.
How To
Determine if a number is prime.
- Step 1. Test each of the primes, in order, to see if it is a factor of the number.
- Step 2. Start with $2$ and stop when the quotient is smaller than the divisor or when a prime factor is found.
- Step 3. If the number has a prime factor, then it is a composite number. If it has no prime factors, then the number is prime.
Example 2.47
Identify each number as prime or composite:
- ⓐ $\phantom{\rule{0.2em}{0ex}}83\phantom{\rule{0.2em}{0ex}}$
- ⓑ $\phantom{\rule{0.2em}{0ex}}77$
Identify the number as prime or composite:
$91$
Identify the number as prime or composite:
$137$
Links To Literacy
Media Access Additional Online Resources
Section 2.4 Exercises
Practice Makes Perfect
Identify Multiples of Numbers
In the following exercises, list all the multiples less than $50$ for the given number.
$3$
$5$
$7$
$9$
$12$
Use Common Divisibility Tests
In the following exercises, use the divisibility tests to determine whether each number is divisible by $2,3,4,5,6,\text{and}\phantom{\rule{0.2em}{0ex}}10.$
$96$
$78$
$264$
$800$
$942$
$750$
$550$
$1080$
$\mathrm{39,075}$
Find All the Factors of a Number
In the following exercises, find all the factors of the given number.
$42$
$48$
$200$
$576$
Identify Prime and Composite Numbers
In the following exercises, determine if the given number is prime or composite.
$67$
$53$
$119$
$221$
$359$
$1771$
Everyday Math
Banking Frank’s grandmother gave him $\text{\$100}$ at his high school graduation. Instead of spending it, Frank opened a bank account. Every week, he added $\text{\$15}$ to the account. The table shows how much money Frank had put in the account by the end of each week. Complete the table by filling in the blanks.
Weeks after graduation | Total number of dollars Frank put in the account | Simplified Total |
---|---|---|
$0$ | $100$ | $100$ |
$1$ | $100+15$ | $115$ |
$2$ | $100+15\cdot 2$ | $130$ |
$3$ | $100+15\cdot 3$ | |
$4$ | $100+15\cdot \left[\phantom{\rule{0.2em}{0ex}}\right]$ | |
$5$ | $100+\left[\phantom{\rule{0.2em}{0ex}}\right]$ | |
$6$ | ||
$20$ | ||
$x$ |
Banking In March, Gina opened a Christmas club savings account at her bank. She deposited $\text{\$75}$ to open the account. Every week, she added $\text{\$20}$ to the account. The table shows how much money Gina had put in the account by the end of each week. Complete the table by filling in the blanks.
Weeks after opening the account | Total number of dollars Gina put in the account | Simplified Total |
---|---|---|
$0$ | $75$ | $75$ |
$1$ | $75+20$ | $95$ |
$2$ | $75+20\cdot 2$ | $115$ |
$3$ | $75+20\cdot 3$ | |
$4$ | $75+20\cdot \left[\phantom{\rule{0.2em}{0ex}}\right]$ | |
$5$ | $75+\left[\phantom{\rule{0.2em}{0ex}}\right]$ | |
$6$ | ||
$20$ | ||
$x$ |
Writing Exercises
If a number is divisible by $2$ and by $3,$ why is it also divisible by $6?$
What is the difference between prime numbers and composite numbers?
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?