6.1: Understanding Percent
- Define and calculate percent.
- Convert between percent, decimal, and fractional values.
- Calculate the total, percent, or part.
- Solve application problems involving percents.
In 2020, the U.S. federal government budgeted $3.5 billion for the National Park Service , which appears to be a very large number (and is!) and a large portion of the total federal budget. However, the total outlays from the U.S. federal government in 2020 was $6.6 trillion . So, the amount budgeted for the National Park Service was less than one-tenth of 1 percent, or 1/10%, of the total outlays. This percent describes a specific number. Understanding that ratio puts the $3.5 billion budgeted to the National Park Service in perspective.
This chapter focuses on percent as a primary tool for understanding money management. The interest paid on debt, the interest earned through investments, and even taxes are entirely determined using percent. This section introduces the basics of working with this invaluable tool.
Define and Calculate Percent
The word percent comes from the Latin phrase per centum, which means "by the hundred." So any percent is a number divided by 100. Changing a percent to a fraction is to write the percent in its fractional form. To write \(n \%\) in its fractional form is to write the percent as the fraction \(\frac{n}{100}\).
A percent need not be an integer and does not have to be less than 100.
Rewrite the following as fractions:
- 18%
- 84%
- 38.7%
- 213%
- Answer
-
- Using the definition and = 18, 18% in fractional form is .
- Using the definition and = 84, 84% in fractional form is .
- Using the definition and = 38.7, 38.7% in fractional form is .
- Using the definition and = 213, 213% in fractional form is .
Rewrite the following as fractions:
- 3%
- 94%
- 67.2%
- 670%
Convert Between Percent, Decimal, and Fractional Values
When any calculation with a percent is to be performed, the form of the percent must be changed, either to its fractional form or its decimal form . We can change a percent into decimal form by dividing the percent by 100 and representing the result as a decimal.
The decimal form of
% is found by calculating the decimal value of
Convert the following percents to decimal form:
- 17%
- 7%
- 18.45%
- Answer
-
- To convert 17% to its decimal form, divide 17 by 100. This moves the decimal two places to the left, resulting in 0.17. The decimal form of 17% is 0.17.
- To convert 7% to its decimal form, divide 7 by 100. This moves the decimal two places to the left, resulting in 0.07. The decimal form of 7% is 0.07.
- To convert 18.45% to its decimal form, divide 18.45 by 100. This moves the decimal two places to the left, resulting in 0.1845. The decimal form of 18.45% is 0.1845.
Convert the following percents to decimal form:
- 9%
- 24%
- 2.18%
You should notice that, to convert from percent to decimal form, you can simply move the decimal two places to the left without performing the division.
To convert the number \(x\) from decimal form to percent, multiply \(x\) by 100 and place a percent sign, %, after the number, .
Convert each of the following to percent:
- 0.34
- 4.15
- 0.0391
- Answer
-
- Using the formula and \(x\) = 0.34, we calculate , which gives us 34%.
- Using the formula and \(x\) = 4.15, we calculate , which gives us 415%.
- Using the formula and \(x\) = 0.0391, we calculate , which gives us 3.91%.
Convert the following to percent:
- 0.41
- 0.02
- 9.2481
You should notice that, to convert from decimal form to percent form, you can simply move the decimal two places to the right without performing the multiplication.
Calculate the Total, Percent, or Part
The word “of” is used to indicate multiplication using fractions, as in “one-fourth of 56.” To find “one-fourth of 56” we would multiply 56 by one-fourth. We can think of percents as fractions with a specific denominator—100. So, to calculate “25% of 52,” we multiply 52 by 25%. But, first we need to convert the percent to either fractional form (25/100) or decimal form. Using the decimal form of 25% we have 0.25 × 52, which equals 13.
In this problem, 52 is the total or base , 25 is the percentage , and 13 is the percentage of 52, or the part of 52. This is sometimes referred to as the amount .
The mathematical formula relating the total (base), the percent in decimal form, and the part (amount) is
In all calculations, the percent is expressed in decimal form.
Knowing any two of the values in our formula allows us to calculate the third value. In the following example, we know the total and the percent, and are asked to find the percentage of the total.
- Determine 70% of 3,500
- Determine 156% of 720
- Answer
-
1. The total is \(x=3,500\), and the percent is \(n=70\). The decimal form of \(70 \%\) is 0.70 . To find the part, or percent of the total, substitute those values into the formula and calculate.
\[
\begin{aligned}
\text { part } & =\text { percent } \times \text { total } \\
& =0.70 \times 3500 \\
& =2450
\end{aligned}
\nonumber \]From this, we say that \(70 \%\) of 3,500 is 2,450 .
2. The total is \(x=720\), and the percent is \(n=156\). The decimal form of \(156 \%\) is 1.56 . To find the part, or percent of the total, substitute those values into the formula and calculate.\[
\begin{aligned}
\text { part } & =\text { percent } \times \text { total } \\
& =1.56 \times 720 \\
& =1,123.2
\end{aligned}
\nonumber \]From this, we say that \(156 \%\) of 720 is \(1,123.2\).
- Determine 26% of 1,300.
- Determine 225% of 915.
Finding Percent of a Total
In the previous example, we knew the total and the percent and found the part using our formula. We may instead know the percent and the part, but not the total. We can use our formula again to solve for the total.
- What is the total if 35% of the total is 70?
- What is the total if 10% of the total is 4,000?
- Answer
-
1. Step 1 : The percent is 35 , which in decimal form is 0.35 . We were given that \(35 \%\) of the total is 70 , so the part is 70 . We are to find the total. Substituting into the formula, we have
<\[
\begin{aligned}
\text { part } & =\text { percent } \times \text { total } \\
70 & =0.35 \times \text { total }
\end{aligned}
\nonumber \]Step 2 : To find the total, we solve the equation for the total.
\[
\begin{aligned}
70 & =0.35 \times \text { total } \\
\frac{70}{0.35} & =\frac{0.35 \times \text { total }}{0.35} \\
200 & =\frac{0.35 \times \text { total }}{0.35} \\
200 & =\text { total }
\end{aligned}
\nonumber \]From this we see that 200 is the total, or, that \(35 \%\) of 200 is 70.
2. Step 1 : The percent is 10 , which in decimal form is 0.1 . We were given that \(10 \%\) of the total is 4,000 , so the part is 4,000 . Substituting into the formula, we have
\[
\begin{aligned}
\text { part } & =\text { percent } \times \text { total } \\
4,000 & =0.1 \times \text { total }
\end{aligned}
\nonumber \]Step 2 : To find the total, we solve the equation for the total.
\[
begin{aligned}
4,000 & =0.1 \times \text { total } \\
\frac{4,000}{0.1} & =\frac{0.1 \times \text { total }}{0.1} \\
40,000 & =\frac{\rho .1 \times \text { total }}{\rho .1} \\
40,000 & =\text { total }
\end{aligned}
\nonumber \]From this we see that 40,000 is the total, or that \(10 \%\) of 40,000 is 4,000 .
What is the total if 18% of the total is 45?
What is the total if 15% of the total is 900?
Finding the Total from the Percent and the Part
Similarly, the percent can be found if the total and the percent of the total (the part) are known. This will result in the decimal form of the percent, so it must be converted to percent form.
- What percent of 500 is 175?
- What percent of 228 is 155?
- Answer
-
1. Step 1: The total is 500 , the percent of the total is 175 . Substituting into the formula, we have
\[
\begin{aligned}
\text { part } & =\text { percent } \times \text { total } \\
175 & =\text { percent } \times 500
\end{aligned}
\nonumber \]Step 2: To find the percent, we solve the equation for the percent.
\[
\begin{aligned}
175 & =\text { percent } \times 500 \\
\frac{175}{500} & =\frac{\text { percent } \times 500}{500} \\
0.35 & =\frac{\text { percent } \times 500}{500} \\
0.35 & =\text { percent }
\end{aligned}
\nonumber \]We see the percent in decimal form is 0.35 . Converting from the decimal form yields \(35 \%\). We say that 175 is \(35 \%\) of 500 .
2. Step 1: The total is 228 , the percent of the total is 155 . Substituting into the formula, we have
\[
\begin{aligned}
\text { part } & =\text { percent } \times \text { total } \\
155 & =\text { percent } \times 228
\end{aligned}
\nonumber \]Step 2: To find the percent, we solve the equation for the percent.
\[
\begin{aligned}
155 & =\text { percent } \times 228 \\
\frac{155}{228} & =\frac{\text { percent } \times 228}{228} \\
0.6798 & =\frac{\text { percent } \times 228}{228} \\
0.6798 & =\text { percent }
\end{aligned}
\nonumber \]
We see the percent is 0.6798 (rounded to four decimal places). Converting from the decimal form yields 67.98%. We say that 155 is 67.98% of 228.
Find the percent in the following:
- Total is 40, percent of the total is 25
- Total is 730, percent of the total is 292
Finding the Percent When the Total and the Part Are Known
Solve Application Problems Involving Percents
Percents are frequently used in finance, research, science experiments, and even casual conversation. Understanding these types of values helps when consuming media or discussing finances, for instance. Effectively working with and interpreting numbers and percents will help you become an informed consumer of this information.
In most cases, working through what is presented requires you to identify that you are indeed working with a question of percents, which two of the three values that are related through percents are known, and which of the three values you need to find.
Justine applies to a medium size university outside her hometown and finds out that the retention rate (percent of students who return for their sophomore year) for the 2021 academic year at the university was 84%. During a visit to the registrar’s office, she finds out that 1,350 people had enrolled in academic year 2021. How many students from the academic year 2021 are returning for the 2022 academic year?
- Answer
-
The percent of students who will return for the 2022 academic year (the retention rate) is 84%. The total number of students who enrolled in the 2021 academic year was 1,350. This means the percent is known and the total is known. From this, we can determine the number of students who will return (percent of the total) for the 2022 academic year using the formula
percent × total part . Substituting into the formula and calculating, we find that the number of students that are returning is= percent × total So 1,134 students will return for the 2022 academic year.
Harris works the bookstore in their hometown. During one particular day, the store had total sales of $1,765, of which Harris sold 30%. What were Harris’s total sales that day?
In this situation, the percent is to be determined. We know the total number of students, 45 , and the part of the students that are chemistry majors, 18 . Using that information and the formula
\[
\text { part }=\text { percent } \times \text { total }
\nonumber \]
, the percent can be found. Substituting and solving, we have
\[
\begin{aligned}
18 & =\text { percent } \times 45 \\
\frac{18}{45} & =\frac{\text { percent } \times 45}{45} \\
0.4 & =\frac{\text { percent } \times 45}{45} \\
0.4 & =\text { percent }
\end{aligned}
\nonumber \]
Converting the 0.4 from decimal form, we find that \(40 \%\) of the students in the calculus class are chemistry majors.
At the Fremont County fair, there were 2,532 adult visitors. Of these, 1,679 purchased the Adult Mega Pass. What percent of the adult visitors purchased the Adult Mega Pass?
Mariel makes a 20% commission on every sale she makes. One week, her commission check is for $153.00. What were her total sales that week?
- Answer
-
In this problem, Mariel's total sales is to be determined. We know the percent she earns is \(20 \%\). We also know that her sales commission was \(\$ 153.00\), which is the percent of the total. Using this information and the formula
\[
\text { part }=\text { percent } \times \text { total }
\nonumber \]we can find Mariel's total sales. The decimal form of \(20 \%\) is 0.2 . The part, or percent of the total, is 153. Substituting and solving, we obtain
\[
\begin{aligned}
\text { part } & =\text { percent } \times \text { total } \\
153 & =0.2 \times \text { total } \\
\frac{153}{0.2} & =\frac{0.2 \times \text { total }}{0.2} \\
765 & =\frac{0.2 \times \text { total }}{0.2}
\end{aligned}
\nonumber \]Mariel’s total sales were $765.00.
Mina’s family has replaced 65% of their home’s older light bulbs with LED bulbs. If they now have 52 LED bulbs, how many total lightbulbs are in Mina’s house?
According to the energy website from the U.S. government, LED lightbulbs use at least 75% less energy than incandescent bulbs. They also last up to 25 times as long as an incandescent bulb. If lighting is a significant percent of your electrical use, replacing all incandescent bulbs with LED bulbs will significantly reduce your electric bill.
Check Your Understanding
1. What is the denominator for any percent?
2. Convert 38.7% to decimal form.
3. What is 68% of 280?
4. Find the total if 41% of the total is 342. If necessary, round to two decimal places.
5. TikTok has an estimated 80,000,000 (80 million) registered users in the United States. The population of the United States is 332,403,650. What percent of the U.S. population are registered TikTok users? If necessary, round to two decimal places.
6. An Amazon fulfillment center needs to hire 20% more drivers. If there are currently 80 drivers, how many more drivers will be hired?