Prealgebra

6.1Understand Percent

Prealgebra6.1 Understand Percent

Learning Objectives

By the end of this section, you will be able to:
• Use the definition of percent
• Convert percents to fractions and decimals
• Convert decimals and fractions to percents
Be Prepared 6.1

Before you get started, take this readiness quiz.

1. Translate “the ratio of $3333$ to $5”5”$ into an algebraic expression.
If you missed this problem, review Table 2.7.
2. Write $3535$ as a decimal.
If you missed this problem, review Example 5.28.
3. Write $0.620.62$ as a fraction.
If you missed this problem, review Example 5.4.

Use the Definition of Percent

How many cents are in one dollar? There are $100100$ cents in a dollar. How many years are in a century? There are $100100$ years in a century. Does this give you a clue about what the word “percent” means? It is really two words, “per cent,” and means per one hundred. A percent is a ratio whose denominator is $100.100.$ We use the percent symbol $%,%,$ to show percent.

Percent

A percent is a ratio whose denominator is $100.100.$

According to data from the American Association of Community Colleges $(2015),(2015),$ about $57%57%$ of community college students are female. This means $5757$ out of every $100100$ community college students are female, as Figure 6.2 shows. Out of the $100100$ squares on the grid, $5757$ are shaded, which we write as the ratio $57100.57100.$

Figure 6.2 Among every $100100$ community college students, $5757$ are female.

Similarly, $25%25%$ means a ratio of $25100,3%25100,3%$ means a ratio of $31003100$ and $100%100%$ means a ratio of $100100.100100.$ In words, "one hundred percent" means the total $100%100%$ is $100100,100100,$ and since $100100=1,100100=1,$ we see that $100%100%$ means $11$ whole.

Example 6.1

According to the Public Policy Institute of California $(2010),44%(2010),44%$ of parents of public school children would like their youngest child to earn a graduate degree. Write this percent as a ratio.

Try It 6.1

Write the percent as a ratio.

According to a survey, $89%89%$ of college students have a smartphone.

Try It 6.2

Write the percent as a ratio.

A study found that $72%72%$ of U.S. teens send text messages regularly.

Example 6.2

In $2007,2007,$ according to a U.S. Department of Education report, $2121$ out of every $100100$ first-time freshmen college students at $4-year4-year$ public institutions took at least one remedial course. Write this as a ratio and then as a percent.

Try It 6.3

Write as a ratio and then as a percent: The American Association of Community Colleges reported that $6262$ out of $100100$ full-time community college students balance their studies with full-time or part time employment.

Try It 6.4

Write as a ratio and then as a percent: In response to a student survey, $4141$ out of $100100$ Santa Ana College students expressed a goal of earning an Associate's degree or transferring to a four-year college.

Convert Percents to Fractions and Decimals

Since percents are ratios, they can easily be expressed as fractions. Remember that percent means per $100,100,$ so the denominator of the fraction is $100.100.$

How To

Convert a percent to a fraction.

1. Step 1. Write the percent as a ratio with the denominator $100.100.$
2. Step 2. Simplify the fraction if possible.

Example 6.3

Convert each percent to a fraction:

1. $36%36%$
2. $125%125%$
Try It 6.5

Convert each percent to a fraction:

1. $48%48%$
2. $110%110%$
Try It 6.6

Convert each percent to a fraction:

1. $64%64%$
2. $150%150%$

The previous example shows that a percent can be greater than $1.1.$ We saw that $125%125%$ means $125100,125100,$ or $54.54.$ These are improper fractions, and their values are greater than one.

Example 6.4

Convert each percent to a fraction:

1. $24.5%24.5%$
2. $3313%3313%$
Try It 6.7

Convert each percent to a fraction:

1. $64.4%64.4%$
2. $6623%6623%$
Try It 6.8

Convert each percent to a fraction:

1. $42.5%42.5%$
2. $834%834%$

In Decimals, we learned how to convert fractions to decimals. To convert a percent to a decimal, we first convert it to a fraction and then change the fraction to a decimal.

How To

Convert a percent to a decimal.

1. Step 1. Write the percent as a ratio with the denominator $100.100.$
2. Step 2. Convert the fraction to a decimal by dividing the numerator by the denominator.

Example 6.5

Convert each percent to a decimal:

1. $6%6%$
2. $78%78%$
Try It 6.9

Convert each percent to a decimal:

1. $9%9%$
2. $87%87%$
Try It 6.10

Convert each percent to a decimal:

1. $3%3%$
2. $91%91%$

Example 6.6

Convert each percent to a decimal:

1. $135%135%$
2. $12.5%12.5%$
Try It 6.11

Convert each percent to a decimal:

1. $115%115%$
2. $23.5%23.5%$
Try It 6.12

Convert each percent to a decimal:

1. $123%123%$
2. $16.8%16.8%$

Let's summarize the results from the previous examples in Table 6.1, and look for a pattern we could use to quickly convert a percent number to a decimal number.

Percent Decimal
$6%6%$ $0.060.06$
$78%78%$ $0.780.78$
$135%135%$ $1.351.35$
$12.5%12.5%$ $0.1250.125$
Table 6.1

Do you see the pattern?

To convert a percent number to a decimal number, we move the decimal point two places to the left and remove the $%%$ sign. (Sometimes the decimal point does not appear in the percent number, but just like we can think of the integer $66$ as $6.0,6.0,$ we can think of $6%6%$ as $6.0%.6.0%.$) Notice that we may need to add zeros in front of the number when moving the decimal to the left.

Figure 6.3 uses the percents in Table 6.1 and shows visually how to convert them to decimals by moving the decimal point two places to the left.

Figure 6.3

Example 6.7

Among a group of business leaders, $77%77%$ believe that poor math and science education in the U.S. will lead to higher unemployment rates.

Convert the percent to: a fraction a decimal

Try It 6.13

Convert the percent to: a fraction and a decimal

Twitter's share of web traffic jumped $24%24%$ when one celebrity tweeted live on air.

Try It 6.14

Convert the percent to: ⓐ a fraction and ⓑ a decimal

The U.S. Census estimated that in $2013,44%2013,44%$ of the population of Boston age $2525$ or older have a bachelor's or higher degrees.

Example 6.8

There are four suits of cards in a deck of cards—hearts, diamonds, clubs, and spades. The probability of randomly choosing a heart from a shuffled deck of cards is $25%.25%.$ Convert the percent to:

1. a fraction
2. a decimal
Figure 6.4 (credit: Riles32807, Wikimedia Commons)
Try It 6.15

Convert the percent to: a fraction, and a decimal

The probability that it will rain Monday is $30%.30%.$

Try It 6.16

Convert the percent to: a fraction, and a decimal

The probability of getting heads three times when tossing a coin three times is $12.5%.12.5%.$

Convert Decimals and Fractions to Percents

To convert a decimal to a percent, remember that percent means per hundred. If we change the decimal to a fraction whose denominator is $100,100,$ it is easy to change that fraction to a percent.

How To

Convert a decimal to a percent.

1. Step 1. Write the decimal as a fraction.
2. Step 2. If the denominator of the fraction is not $100,100,$ rewrite it as an equivalent fraction with denominator $100.100.$
3. Step 3. Write this ratio as a percent.

Example 6.9

Convert each decimal to a percent: $0.050.05$ $0.830.83$

Try It 6.17

Convert each decimal to a percent: $0.010.01$ $0.17.0.17.$

Try It 6.18

Convert each decimal to a percent: $0.040.04$ $0.410.41$

To convert a mixed number to a percent, we first write it as an improper fraction.

Example 6.10

Convert each decimal to a percent: $1.051.05$ $0.0750.075$

Try It 6.19

Convert each decimal to a percent: $1.751.75$ $0.08250.0825$

Try It 6.20

Convert each decimal to a percent: $2.252.25$ $0.09250.0925$

Let's summarize the results from the previous examples in Table 6.2 so we can look for a pattern.

Decimal Percent
$0.050.05$ $5%5%$
$0.830.83$ $83%83%$
$1.051.05$ $105%105%$
$0.0750.075$ $7.5%7.5%$
Table 6.2

Do you see the pattern? To convert a decimal to a percent, we move the decimal point two places to the right and then add the percent sign.

Figure 6.5 uses the decimal numbers in Table 6.2 and shows visually to convert them to percents by moving the decimal point two places to the right and then writing the $%%$ sign.

Figure 6.5

In Decimals, we learned how to convert fractions to decimals. Now we also know how to change decimals to percents. So to convert a fraction to a percent, we first change it to a decimal and then convert that decimal to a percent.

How To

Convert a fraction to a percent.

1. Step 1. Convert the fraction to a decimal.
2. Step 2. Convert the decimal to a percent.

Example 6.11

Convert each fraction or mixed number to a percent: $3434$ $118118$ $215215$

Try It 6.21

Convert each fraction or mixed number to a percent: $5858$ $114114$ $325325$

Try It 6.22

Convert each fraction or mixed number to a percent: $7878$ $9494$ $135135$

Sometimes when changing a fraction to a decimal, the division continues for many decimal places and we will round off the quotient. The number of decimal places we round to will depend on the situation. If the decimal involves money, we round to the hundredths place. For most other cases in this book we will round the number to the nearest thousandth, so the percent will be rounded to the nearest tenth.

Example 6.12

Convert $5757$ to a percent.

Try It 6.23

Convert the fraction to a percent: $3737$

Try It 6.24

Convert the fraction to a percent: $4747$

When we first looked at fractions and decimals, we saw that fractions converted to a repeating decimal. When we converted the fraction $4343$ to a decimal, we wrote the answer as $1.3¯.1.3¯.$ We will use this same notation, as well as fraction notation, when we convert fractions to percents in the next example.

Example 6.13

An article in a medical journal claimed that approximately $1313$ of American adults are obese. Convert the fraction $1313$ to a percent.

Try It 6.25

Convert the fraction to a percent:

According to the U.S. Census Bureau, about $1919$ of United States housing units have just $11$ bedroom.

Try It 6.26

Convert the fraction to a percent:

According to the U.S. Census Bureau, about $1616$ of Colorado residents speak a language other than English at home.

Section 6.1 Exercises

Practice Makes Perfect

Use the Definition of Percents

In the following exercises, write each percent as a ratio.

1.

In $2014,2014,$ the unemployment rate for those with only a high school degree was $6.0%.6.0%.$

2.

In $2015,2015,$ among the unemployed, $29%29%$ were long-term unemployed.

3.

The unemployment rate for those with Bachelor's degrees was $3.2%3.2%$ in $2014.2014.$

4.

The unemployment rate in Michigan in $20142014$ was $7.3%.7.3%.$

In the following exercises, write as

1. a ratio and
2. a percent
5.

$5757$ out of $100100$ nursing candidates received their degree at a community college.

6.

$8080$ out of $100100$ firefighters and law enforcement officers were educated at a community college.

7.

$4242$ out of $100100$ first-time freshmen students attend a community college.

8.

$7171$ out of $100100$ full-time community college faculty have a master's degree.

Convert Percents to Fractions and Decimals

In the following exercises, convert each percent to a fraction and simplify all fractions.

9.

$4%4%$

10.

$8%8%$

11.

$17%17%$

12.

$19%19%$

13.

$52%52%$

14.

$78%78%$

15.

$125%125%$

16.

$135%135%$

17.

$37.5%37.5%$

18.

$42.5%42.5%$

19.

$18.4%18.4%$

20.

$46.4%46.4%$

21.

$912%912%$

22.

$812%812%$

23.

$513%513%$

24.

$623%623%$

In the following exercises, convert each percent to a decimal.

25.

$5%5%$

26.

$9%9%$

27.

$1%1%$

28.

$2%2%$

29.

$63%63%$

30.

$71%71%$

31.

$40%40%$

32.

$50%50%$

33.

$115%115%$

34.

$125%125%$

35.

$150%150%$

36.

$250%250%$

37.

$21.4%21.4%$

38.

$39.3%39.3%$

39.

$7.8%7.8%$

40.

$6.4%6.4%$

In the following exercises, convert each percent to

1. a simplified fraction and
2. a decimal
41.

In $2010,1.5%2010,1.5%$ of home sales had owner financing. (Source: Bloomberg Businessweek, 5/23–29/2011)

42.

In $2000,4.2%2000,4.2%$ of the United States population was of Asian descent. (Source: www.census.gov)

43.

According to government data, in $20132013$ the number of cell phones in India was $70.23%70.23%$ of the population.

44.

According to the U.S. Census Bureau, among Americans age $2525$ or older who had doctorate degrees in $2014,37.1%2014,37.1%$ are women.

45.

A couple plans to have two children. The probability they will have two girls is $25%.25%.$

46.

Javier will choose one digit at random from $00$ through $9.9.$ The probability he will choose $33$ is $10%.10%.$

47.

According to the local weather report, the probability of thunderstorms in New York City on July $1515$ is $60%.60%.$

48.

A club sells $5050$ tickets to a raffle. Osbaldo bought one ticket. The probability he will win the raffle is $2%.2%.$

Convert Decimals and Fractions to Percents

In the following exercises, convert each decimal to a percent.

49.

$0.010.01$

50.

$0.030.03$

51.

$0.180.18$

52.

$0.150.15$

53.

$1.351.35$

54.

$1.561.56$

55.

$33$

56.

$44$

57.

$0.0090.009$

58.

$0.0080.008$

59.

$0.08750.0875$

60.

$0.06250.0625$

61.

$1.51.5$

62.

$2.22.2$

63.

$2.2542.254$

64.

$2.3172.317$

In the following exercises, convert each fraction to a percent.

65.

$1414$

66.

$1515$

67.

$3838$

68.

$5858$

69.

$7474$

70.

$9898$

71.

$645645$

72.

$514514$

73.

$512512$

74.

$11121112$

75.

$223223$

76.

$123123$

77.

$3737$

78.

$6767$

79.

$5959$

80.

$4949$

In the following exercises, convert each fraction to a percent.

81.

$1414$ of washing machines needed repair.

82.

$1515$ of dishwashers needed repair.

In the following exercises, convert each fraction to a percent.

83.

According to the National Center for Health Statistics, in $2012,7202012,720$ of American adults were obese.

84.

The U.S. Census Bureau estimated that in $2013,85%2013,85%$ of Americans lived in the same house as they did $11$ year before.

In the following exercises, complete the table.

85.
Fraction Decimal Percent
$1212$
$0.450.45$
$18%18%$
$1313$
$0.00080.0008$
$22$
Table 6.3
86.
Fraction Decimal Percent
$1414$
$0.650.65$
$22%22%$
$2323$
$0.00040.0004$
$33$
Table 6.4

Everyday Math

87.

Sales tax Felipa says she has an easy way to estimate the sales tax when she makes a purchase. The sales tax in her city is $9.05%.9.05%.$ She knows this is a little less than $10%.10%.$

1. Convert $10%10%$ to a fraction.
2. Use your answer from to estimate the sales tax Felipa would pay on a $9595$ dress.
88.

Savings Ryan has $25%25%$ of each paycheck automatically deposited in his savings account.

1. Write $25%25%$ as a fraction.
2. Use your answer from to find the amount that goes to savings from Ryan's $2,4002,400$ paycheck.
89.

Amelio is shopping for textbooks online. He found three sellers that are offering a book he needs for the same price, including shipping. To decide which seller to buy from he is comparing their customer satisfaction ratings. The ratings are given in the chart.

Seller Rating
$AA$ $4/54/5$
$BB$ $3.5/43.5/4$
$CC$ $85%85%$
90.

Write seller $C’sC’s$ rating as a fraction and a decimal.

91.

Write seller $B’sB’s$ rating as a percent and a decimal.

92.

Write seller $A’sA’s$ rating as a percent and a decimal.

93.

Which seller should Amelio buy from and why?

Writing Exercises

94.

Convert $25%,50%,75%,and100%25%,50%,75%,and100%$ to fractions. Do you notice a pattern? Explain what the pattern is.

95.

Convert $110,210,310,410,510,610,710,810,110,210,310,410,510,610,710,810,$ and $910910$ to percents. Do you notice a pattern? Explain what the pattern is.

96.

When the Szetos sold their home, the selling price was $500%500%$ of what they had paid for the house $30 years30 years$ ago. Explain what $500%500%$ means in this context.

97.

According to cnn.com, cell phone use in $20082008$ was $600%600%$ of what it had been in $2001.2001.$ Explain what $600%600%$ means in this context.

Self Check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

If most of your checks were:

…confidently. Congratulations! You have achieved the objectives in this section. Reflect on the study skills you used so that you can continue to use them. What did you do to become confident of your ability to do these things? Be specific.

…with some help. This must be addressed quickly because topics you do not master become potholes in your road to success. In math, every topic builds upon previous work. It is important to make sure you have a strong foundation before you move on. Who can you ask for help? Your fellow classmates and instructor are good resources. Is there a place on campus where math tutors are available? Can your study skills be improved?

…no—I don’t get it! This is a warning sign and you must not ignore it. You should get help right away or you will quickly be overwhelmed. See your instructor as soon as you can to discuss your situation. Together you can come up with a plan to get you the help you need.