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Mathematics LibreTexts

1.5: Review of Decimals and Percents

  • Page ID
    18330
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    Skills to Develop

    • Convert fractions to decimals and back.
    • Perform operations with decimals.
    • Round off decimals to a given place.
    • Define a percent.
    • Convert percents to decimals and back.
    • Convert fractions to percents and back.

    Decimals

    In this section, we provide a brief review of the decimal system. A real number in decimal form, a decimal consists of a decimal point, digits (\(0\) through \(9\)) to the left of the decimal point representing the whole number part, and digits to the right of the decimal point representing the fractional part. The digits represent powers of 10 as shown in the set \(\{…, 1,000, 100, 10, 1, 1/10, 1/100, 1/1,000, …\} \)according to the following diagram:

    https://2012books.lardbucket.org/books/beginning-algebra/section_04/e610f3d2400682e1148d7aa32aaa8ea3.jpg

    Figure 1.5.1

    For example, the decimal \(538.3\) can be written in the following expanded form:

    \(5 \cdot \color{Cerulean}{100} \color{Black}{+} 3 \cdot \color{Cerulean}{10} \color{Black}{+} 8 \cdot \color{Cerulean}{1} \color{Black}{+} 3 \cdot \color{Cerulean}{\frac{1}{10}} \)

    After simplifying, we obtain the mixed number \(528 \frac{3}{10}\). Use this process to convert decimals to mixed numbers.

    Example \(\PageIndex{1}\)

    Write as a mixed number:

    \(32.15\).

    Solution:

    In this example, \(32\) is the whole part and the decimal ends in the hundredths place. Therefore, the fractional part will be \(\frac{15}{100}\), and we can write

    \(\begin{aligned} 32.15&=32+\frac{15}{100}&\color{Cerulean}{Reduce.} \\ &=32+\frac{3}{20} \\ &=32\frac{3}{20} \end{aligned}\)

    Answer:

    \(32.15=32\frac{3}{20}\)

    To convert fractions to decimal equivalents, divide.

    Example \(\PageIndex{2}\)

    Write as a decimal:

    \(\frac{3}{4}\).

    Solution:

    Use long division to convert to a decimal.

    Screenshot (955).png

    Answer:

    \(\frac{3}{4}=0.75\)

    If the division never ends, then use a bar over the repeating digit (or block of digits) to indicate a repeating decimal.

    Example \(\PageIndex{3}\)

    Write as a decimal:

    \(2\frac{5}{6}\).

    Solution:

    Use long division to convert the fractional part to a decimal and then add the whole part.

    Screenshot (956).png

    At this point, we can see that the long division will continue to repeat. When this is the case, use a bar over the repeating digit to indicate that it continues forever:

    \(\frac{5}{6}=0.8\overline{3}\)

    Then write

    \(\begin{aligned} 2\frac{5}{6}&=2+\frac{5}{6} \\ &=2+0.8\overline{3} \\ &=2.8\overline{3} \end{aligned}\)

    Answer:

    \(2\frac{5}{6}=2.8\overline{3}\)

    To add or subtract decimals, align them vertically with the decimal point and add corresponding place values. Recall that sometimes you need to borrow from or carry over to the adjoining column (regrouping).

    Example \(\PageIndex{4}\)

    Subtract:

    \(54.328−23.25\).

    Solution:

    Note that trailing zeros to the right of the decimal point do not change the value of the decimal, \(23.25 = 23.250\). In this case, you need to borrow from the tenths place (regroup) to subtract the digits in the hundredths place.

    Screenshot (957).png

    Answer:

    \(31.078\)

    Multiply decimals the same way you multiply whole numbers. The number of decimal places in the product will be the sum of the decimal places found in each of the factors.

    Example \(\PageIndex{5}\)

    Multiply:

    \(5.36×7.4\).

    Solution:

    The total number of decimal places of the two factors is \(2 + 1 = 3\). Therefore, the result has \(3\) decimal places.

    Screenshot (958).png

    Answer:

    \(39.664\)

    When dividing decimals, move the decimal points of both the dividend and the divisor so that the divisor is a whole number. Remember to move the decimal the same number of places for both the dividend and divisor.

    Example \(\PageIndex{6}\)

    Divide:

    \(33.3216÷6.24\).

    Solution:

    Move the decimal point to turn the divisor into a whole number: \(624\). Move the decimal points of both the divisor and dividend two places to the right.

    Screenshot (959).png

    Next, divide,

    Answer:

    \(5.34\)

    It is often necessary to round off decimals to a specified number of decimal places. Rounding off allows us to approximate decimals with fewer significant digits. To do this, look at the digit to the right of the specified place value.

    1. If the digit to the right of the specified place is \(4\) or less, then leave the specified digit unchanged and drop all subsequent digits.
    2. If the digit to the right of the specified place is \(5\) or greater, then increase the value of the digit in the specified place by \(1\) and drop all subsequent digits.

    Recall that decimals with trailing zeros to the right of the decimal point can be dropped. For example, round \(5.635457\) to the nearest thousandth:

    \(\begin{aligned}\color{OliveGreen}{Thou}&\color{OliveGreen}{sandth} \\ &\color{OliveGreen}{\downarrow} \\ 5.63&5\color{Cerulean}{4}\color{black}{57}&\color{Cerulean}{Round\:down.} \\ \approx5.63&5 \end{aligned}\)

    Round the same number \(5.635457\) to the nearest hundredth:

    \(\begin{aligned}\color{OliveGreen}{Hun}&\color{OliveGreen}{dredth} \\ &\color{OliveGreen}{\downarrow} \\ 5.6&3\color{Cerulean}{5}\color{black}{457}&\color{Cerulean}{Round\:up.} \\ \approx5.6&4 \end{aligned}\)

    After rounding off, be sure to use the appropriate notation \(( ≈ )\) to indicate that the number is now an approximation. When working with US currency, we typically round off to two decimal places, or the nearest hundredth.

    Example \(\PageIndex{7}\)

    Calculate and round off to the nearest hundredth.

    1. \(\frac{1}{3}\) of $\(10.25\).
    2. \(\frac{1}{4}\) of $\(10.25\).

    Solution:

    In this context, the key word “of” indicates that we should multiply.

    a. Multiplying by \(\frac{1}{3}\) is equivalent to dividing by \(3\).

    \(\begin{aligned} \frac{1}{3}\cdot $10.25 &=\frac{$10.25}{3} \\ &=$3.41\overline{6}&\color{Cerulean}{Round\:up.} \\ &\approx $3.42 \end{aligned}\)

    b. Multiplying by \(\frac{1}{4}\) is equivalent to dividing by \(4\).

    \(\begin{aligned} \frac{1}{4}\cdot $10.25 &=\frac{$10.25}{4} \\ &=$2.5625&\color{Cerulean}{Round\:down.} \\ &\approx $2.56 \end{aligned}\)

    Answer:

    a. $\(3.42\); b. $\(2.56\)

    Definition of Percent

    A percent is a representation of a number as a part of one hundred. The word “percent” can be written “per cent” which means “per \(100\)” or “\(/100\).” We use the symbol ( % ) to denote a percentage:

    \[N\%=\frac{N}{100}\]

    For example,

    \(\begin{array}{cc} {5\%=\frac{5}{100}=0.05}&{100\%=\frac{100}{100}=1}\\{72\%=\frac{72}{100}=0.72}&{135\%=\frac{135}{100}=1.35} \end{array}\)

    Percents are an important part of our everyday life and show up often in our study of algebra. Percents can be visualized using a pie chart (or circle graph), where each sector gives a visual representation of a percentage of the whole. For example, the following pie chart shows the percentage of students in certain age categories of all US community colleges.

    Screenshot (960).png

    Figure 1.5.2: Source: American Association of Community Colleges.

    Each sector is proportional to the size of the part out of the whole. The sum of the percentages presented in a pie chart must be \(100\)%. To work with percentages effectively, you have to know how to convert percents to decimals or fractions and back again.

    Percents to Decimals

    Applying the definition of percent, you see that \(58\%=\frac{58}{100}=0.58\). The same result can be obtained by moving the decimal two places to the left. To convert percents to decimals, either apply the definition or move the decimal two places to the left.

    Example \(\PageIndex{8}\)

    Convert to a decimal:

    \(152\)%.

    Solution:

    Treat \(152\)% as \(152.0\)% and move the decimal two places to the left.

    Screenshot (961).png

    Answer:

    \(1.52\)

    Example \(\PageIndex{9}\)

    Convert to a decimal:

    \(2\frac{3}{4}\)%.

    Solution:

    First, write the decimal percent equivalent,

    \(2\frac{3}{4}\%=2.75\%\)

    Next, move the decimal two places to the left,

    Screenshot (962).png

    At this point, fill in the tenths place with a zero.

    \(2.75\%=0.0275\)

    Answer:

    \(0.0275\)

    Exercise \(\PageIndex{1}\)

    Convert to a decimal:

    \(215\)%.

    Answer

    \(2.15\) 

    Decimals and Fractions to Percents

    To convert a decimal to a percent, convert the decimal to a fraction of 100 and apply the definition of percent, or equivalently, move the decimal to the right two places and add a percent sign.

    Example \(\PageIndex{10}\)

    Convert \(0.23\) to a percent.

    Solution:

    First, convert the decimal to a fraction of 100 and apply the definition.

    \(0.23=\frac{23}{100}=23\%\)

    You can achieve the same result by moving the decimal two places to the right and adding a percent sign.

    Screenshot (963).png

    Answer:

    \(23\)%

    Alternatively, you can multiply by \(1\) in the form of \(100\)%.

    Example \(\PageIndex{11}\)

    Convert \(2.35\) to a percent.

    Solution:

    Recall that \(1 = 100\)%.

    \(\begin{aligned} 2.35&=2.35\cdot 1\\ &=2.35\cdot 100\% \\ &=235\% \end{aligned}\)

    You can achieve the same result by moving the decimal two places to the right and adding a percent sign.

    Screenshot (964).png

    Answer:

    \(235\)%

    Example \(\PageIndex{12}\)

    Convert \(5\frac{1}{5}\) to a percent.

    Solution:

    \(\begin{aligned} 5\frac{1}{5}&=5.20\\ &=520\% \end{aligned}\)

    Answer:

    \(520\)%

    Sometimes we can use the definition of percent and find an equivalent fraction with a denominator of \(100\).

    Example \(\PageIndex{13}\)

    Convert \(\frac{13}{25}\) to a percent.

    Solution:

    Notice that the denominator \(25\) is a factor of \(100\). Use the definition of percent by finding an equivalent fraction with \(100\) in the denominator.

    \(\frac{13}{25}=\color{black}{\frac{13\color{Cerulean}{\cdot 4}}{25\color{Cerulean}{\cdot 4}}=\frac{52}{100}=52\%}\)

    This is a very specialized technique because \(100\) may not be a multiple of the denominator.

    Example \(\PageIndex{14}\)

    Convert \(\frac{1}{3}\) to a percent.

    Solution:

    Notice that the denominator \(3\) is not a factor of \(100\). In this case, it is best to multiply by \(1\) in the form of \(100\)%.

    \(\begin{aligned}\frac{1}{3}&=\frac{1}{3}\cdot 100\% \\ &=\frac{100}{2}\% \\ &=33\frac{1}{3}\% \end{aligned}\)

    Answer:

    \(33\frac{1}{3}\%\)

    Exercise \(\PageIndex{2}\)

    Convert to a percent:

    \(\frac{2}{3}\)

    Answer

    \(66\frac{2}{3}\)%

    Percents to Fractions

    When converting percents to fractions, apply the definition of percent and then reduce.

    Example \(\PageIndex{15}\)

    Convert to a fraction:

    \(28\)%.

    Solution:

    \(\begin{aligned} 28\%&=\frac{28}{100} \\ &=\color{black}{\frac{28\color{Cerulean}{\div 4}}{100\color{Cerulean}{\div 4}}} \\ &=\frac{7}{25} \end{aligned}\)

    Answer:

    \(\frac{7}{25}\)

    Applying the definition of percent is equivalent to removing the percent sign and multiplying by \(\frac{1}{100}\).

    Example \(\PageIndex{16}\)

    Convert to a fraction:

    \(66\frac{2}{3}\)%.

    Solution:

    First, convert to an improper fraction and then apply the definition of percent.

    \(\begin{aligned} 66\frac{2}{3}\%&=\frac{200}{3}\% \\ &=\frac{200}{3}\div 100 \\ &=\frac{\color{Cerulean}{\stackrel{2}{\cancel{\color{black}{200}}}}}{3}\color{black}{\cdot\frac{1}{\color{Cerulean}{\stackrel{\cancel{\color{black}{100}}}{1}}}} \\ &=\frac{2}{3} \end{aligned}\)

    Answer:

    \(\frac{2}{3}\)

    Exercise \(\PageIndex{3}\)

    Convert to a fraction:

    \(3\frac{7}{31}\)%.

    Answer

    \(\frac{1}{31}\)

    Example \(\PageIndex{17}\)

    Using the given pie chart, calculate the total number of students that were \(21\) years old or younger if the total US community college enrollment in 2009 was \(11.7\) million.

    Solution:

    From the pie chart we can determine that \(47\)% of the total \(11.7\) million students were \(21\) years old or younger.

    Screenshot (965).png

    Figure 1.5.3: Source: American Association of Community Colleges.

    Convert \(47\)% to a decimal and multiply as indicated by the key word “of.”

    \(0.47\cdot 11.7\approx 5.5\)

    Answer:

    In 2009, approximately \(5.5\) million students enrolled in US community colleges were \(21\) years old or younger.

    Key Takeaways

    • To convert a decimal to a mixed number, add the appropriate fractional part indicated by the digits to the right of the decimal point to the whole part indicated by the digits to the left of the decimal point and reduce if necessary.
    • To convert a mixed number to a decimal, convert the fractional part of the mixed number to a decimal using long division and then add it to the whole number part.
    • To add or subtract decimals, align them vertically with the decimal point and add corresponding place values.
    • To multiply decimals, multiply as usual for whole numbers and count the number of decimal places of each factor. The number of decimal places in the product will be the sum of the decimal places found in each of the factors.
    • To divide decimals, move the decimal in both the divisor and dividend until the divisor is a whole number and then divide as usual.
    • When rounding off decimals, look to the digit to the right of the specified place value. If the digit to the right is \(4\) or less, round down by leaving the specified digit unchanged and dropping all subsequent digits. If the digit to the right is \(5\) or more, round up by increasing the specified digit by one and dropping all subsequent digits.
    • A percent represents a number as part of \(100\): \(N\%=\frac{N}{100}\).
    • To convert a percent to a decimal, apply the definition of percent and write that number divided by \(100\). This is equivalent to moving the decimal two places to the left.
    • To convert a percent to a fraction, apply the definition of percent and then reduce.
    • To convert a decimal or fraction to a percent, multiply by \(1\) in the form of \(100\)%. This is equivalent to moving the decimal two places to the right and adding a percent sign.
    • Pie charts are circular graphs where each sector is proportional to the size of the part out of the whole. The sum of the percentages must total \(100\)%.

    Exercise \(\PageIndex{4}\) Decimals

    Write as a mixed number.

    1. \(45.8\)
    2. \(15.4\)
    3. \(1.82\)
    4. \(2.55\)
    5. \(4.72\)
    6. \(3.14\)
    Answer

    1. \(45\frac{4}{5}\)

    3. \(1\frac{41}{50}\)

    5. \(4\frac{18}{25}\)

    Exercise \(\PageIndex{5}\) Decimals

    Write as a decimal.

    1. \(2\frac{4}{5}\)
    2. \(5\frac{1}{5}\)
    3. \(3\frac{1}{8}\)
    4. \(1\frac{3}{20}\)
    5. \(\frac{3}{8}\)
    6. \(\frac{5}{8}\)
    7. \(1\frac{1}{3}\)
    8. \(2\frac{1}{6}\)
    Answer

    1. \(2.8\)

    3. \(3.125\)

    5. \(0.375\)

    7. \(1.\overline{3}\)

    Exercise \(\PageIndex{6}\) Decimals

    Perform the operations. Round dollar amounts to the nearest hundredth.

    1. \(13.54−4.6\)
    2. \(16.8−4.845\)
    3. \(45.631+7.82\)
    4. \(256.34+51.771\)
    5. \(12.82×5.9\)
    6. \(123.5×0.17\)
    7. \(0.451×1.5\)
    8. \(0.836×9.3\)
    9. \(38.319÷5.3\)
    10. \(52.6551÷5.01\)
    11. \(0.9338÷0.023\)
    12. \(4.6035÷0.045\)
    13. Find \(\frac{1}{6}\) of \($20.00\).
    14. Find \(\frac{1}{5}\) of \($33.26\).
    15. Find \(\frac{2}{3}\) of \($15.25\).
    16. Find \(\frac{3}{4}\) of \($15.50\).
    Answer

    1. \(8.94\)

    3. \(53.451\)

    5. \(75.638\)

    7. \(0.6765\)

    9. \(7.23\)

    11. \(40.6\)

    13. \($3.33\)

    15. \($10.17\)

    Exercise \(\PageIndex{7}\) Decimals

    1. A gymnast scores \(8.8\) on the vault, \(9.3\) on the uneven bars, \(9.1\) on the balance beam, and \(9.8\) on the floor exercise. What is her overall average?
    2. To calculate a batting average, divide the player’s number of hits by the total number of at-bats and round off the result to three decimal places. If a player has \(62\) hits in \(195\) at-bats, then what is his batting average?
    Answer

    1. \(9.25\)

    Exercise \(\PageIndex{8}\) Percents to Decimals

    Convert each percent to its decimal equivalent.

    1. \(43\)%
    2. \(25\)%
    3. \(33\)%
    4. \(100\)%
    5. \(150\)%
    6. \(215\)%
    7. \(\frac{1}{2}\)%
    8. \(2\frac{3}{4}\)%
    9. \(1\frac{1}{2}\)%
    10. \(3\frac{2}{3}\)%
    11. \(0.025\)%
    12. \(0.0001\)%
    13. \(1.75\)%
    14. \(20.34\)%
    15. \(0\)%
    16. \(1\)%
    17. \(3.05\)%
    18. \(5.003\)%
    19. Convert one-half of one percent to a decimal.
    20. Convert three-quarter percent to a decimal.
    21. What is \(20\)% of zero?
    22. What is \(50\)% of one hundred?
    23. What is \(150\)% of \(100\)?
    24. What is \(20\)% of $\(20.00\)?
    25. What is \(112\)% of $\(210\)?
    26. What is \(9\frac{1}{2}\)% of $\(1,200\)?
    27. If the bill at a restaurant comes to $\(32.50\), what is the amount of a \(15\)% tip?
    28. Calculate the total cost, including a \(20\)% tip, of a meal totaling $\(37.50\).
    29. If an item costs $\(45.25\), then what is the total after adding \(8.25\)% for tax?
    30. If an item costs $\(36.95\), then what is the total after adding \(9\frac{1}{4}\)% tax?
    31. A retail outlet is offering \(15\)% off the original $\(29.99\) price of branded sweaters. What is the price after the discount?
    32. A solar technology distribution company expects a \(12\)% increase in first quarter sales as a result of a recently implemented rebate program. If the first quarter sales last year totaled $\(350,000\), then what are the sales projections for the first quarter of this year?
    33. If a local mayor of a town with a population of \(40,000\) people enjoys a \(72\)% favorable rating in the polls, then how many people view the mayor unfavorably?
    34. If a person earning $\(3,200\) per month spends \(32\)% of his monthly income on housing, then how much does he spend on housing each month?
    Answer

    1. \(0.43\)

    3. \(0.33\)

    5. \(1.5\)

    7. \(0.005\)

    9. \(0.015\)

    11. \(0.00025\)

    13. \(0.0175\)

    15. \(0\)

    17. \(0.0305\)

    19. \(0.005\)

    21. \(0\)

    23. \(150\)

    25. \($235.20\)

    27. \($4.88\)

    29. \($48.98\)

    31. \($25.49\)

    33. \(11,200\) people

    Exercise \(\PageIndex{9}\) Decimals and Fractions to Percents

    Convert the following decimals and fractions to percents.

    1. \(0.67\)
    2. \(0.98\)
    3. \(1.30\)
    4. \(2.25\)
    5. \(\frac{57}{100}\)
    6. \(\frac{99}{100}\)
    7. \(\frac{1}{5}\)
    8. \(\frac{2}{3}\)
    9. \(\frac{25}{8}\)
    10. \(3\frac{1}{4}\)
    11. \(\frac{17}{50}\)
    12. \(\frac{1}{7}\)
    13. \(0.0023\)
    14. \(0.000005\)
    15. \(20\)
    16. \(100\)
    Answer

    1. \(67\)%

    3. \(130\)%

    5. \(57\)%

    7. \(20\)%

    9. \(312.5\)%

    11. \(34\)%

    13. \(0.23\)%

    15. \(2,000\)%

    Exercise \(\PageIndex{10}\) Percents to Fractions

    Use the definition of percent to convert to fractions.

    1. \(20\)%
    2. \(80\)%
    3. \(57\)%
    4. \(97\)%
    5. \(5\frac{1}{2}\)%
    6. \(1\frac{2}{3}\)%
    7. \(75\)%
    8. \(32\)%
    9. \(400\)%
    10. \(230\)%
    11. \(100\)%
    12. \(\frac{1}{8}\)%
    13. \(\frac{5}{12}\)%
    14. \(5\frac{5}{7}\)%
    15. \(33\frac{1}{3}\)%
    16. \(3\frac{7}{31}\)%
    17. \(0.7\)%
    18. \(0.05\)%
    19. \(1.2\)%
    20. \(12.5\)%
    Answer

    1. \(\frac{1}{5}\)

    3. \(\frac{57}{100}\)

    5. \(\frac{11}{200}\)

    7. \(\frac{3}{4}\)

    9. \(4\)

    11. \(1\)

    13. \(\frac{12}{40}\)

    15. \(\frac{1}{3}\)

    17. \(\frac{7}{1000}\)

    19. \(\frac{32}{50}\)

    Exercise \(\PageIndex{11}\) Percents to Fractions

    The course grade weighting for a traditional mathematics course with \(1,200\) total points is shown in the pie chart below. Use the chart to answer the following questions.

    Screenshot (966).png

    Figure 1.5.4

    1. How many points will the final exam be worth?
    2. How many points will the homework be worth?
    3. How many points will each of the four regular exams be worth?
    4. How many \(10\)-point homework assignments can be assigned?
    Answer

    1. \(360\) points

    3. \(180\) points

    Exercise \(\PageIndex{12}\) Percents to Fractions

    A website had \(12,000\) unique users in the fall of 2009. Answer the questions based on the pie chart below depicting total Web browser usage.

    Screenshot (967).png

    Figure 1.5.5

    1. How many users used the Firefox Web browser?
    2. How many users used a browser other than Internet Explorer?
    3. How many users used either Firefox or Internet Explorer?
    4. How many users used Google Chrome or Safari?
    Answer

    1. \(3,000\) users

    3. \(10,560\) users

    Exercise \(\PageIndex{13}\) Percents to Fractions

    The 2009 employment status of \(11.7\) million full-time community college students is given in the following pie chart. Use the chart to answer the following questions. Round off each answer to the nearest hundredth.

    Screenshot (968).png

    Figure 1.5.6: Source: American Association of Community Colleges.

    1. How many full-time students were employed full time?
    2. How many full-time students were employed part time?
    3. How many full-time students were unemployed or employed part time?
    4. How many full-time students also worked part time or full time?
    Answer

    1. \(3.16\) million

    3. \(8.54\) million

    Exercise \(\PageIndex{14}\) Percents to Fractions

    The pie chart below depicts all US households categorized by income. The total number of households in 2007 was about \(111,600,000\). Use the chart to answer the following questions.

    Screenshot (969).png

    Figure 1.5.7: Source: US Census Bureau.​​​​​​​

    1. How many households reported an income from \($50,000\) to \($74,999\)?
    2. How many households reported an income from \($75,000\) to \($99,999\)?
    3. How many households reported an income of \($100,000\) or more?
    4. How many households reported an income of less than \($25,000\)?
    Answer

    1. \(20,980,800\) households

    3. \(21,204,000\) households

    Exercise \(\PageIndex{15}\) Discussion Board Topics

    1. The decimal system is considered a base-10 numeral system. Explain why. What other numeral systems are in use today?
    2. Research and discuss the history of the symbol %.
    3. Research and discuss simple interest and how it is calculated. Make up an example and share the solution.
    4. Discuss methods for calculating tax and total bills.
    5. Research and discuss the history of the pie chart.
    6. Research and discuss the idea of a weighted average
    Answer

    1. Answers may vary

    3. Answers may vary

    5. Answers may vary