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Mathematics LibreTexts

8.1: Percent

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We have to work with money every day. Calculating your monthly expenses, splitting the tab on a restaurant bill, or figuring out how much the tip should be, requires only arithmetic. But when we start saving for the future, planning for retirement, or need a loan, then we need more mathematics.

When one hears the word “percent,” other words come immediately to mind, words such as “century,” “cents,” or “centimeters.” A century equals 100 years. There are one hundred cents in a dollar and there are 100 centimeters in a meter. Thus, it should come as no surprise that percent means “parts per hundred.”

The Meaning of Percent

In the square shown in Figure 8.1.1, a large square has been partitioned into ten rows of ten little squares in each row. We have shaded 20 of 100 possible little squares, or 20% of the total number of little squares.

Screen Shot 2019-09-20 at 3.32.24 PM.png
Figure 8.1.1: Shading 20 of 100 little squares, or 20% of the total number of little squares.

Notice in the figure that 80 out of a possible 100 squares are left unshaded. Thus, 80% of the little squares are unshaded. If instead we shaded 35 out of the 100 squares, then 35% of the little squares would be shaded. If we shaded all of the little squares, then 100% of the little squares would be shaded (100 out of 100).

So, when you hear the word “percent,” think “parts per hundred.”

Changing a Percent to a Fraction

Based on the discussion above, it is fairly straightforward to change a percent to a fraction.

Percent to Fraction

To change a percent to a fraction, drop the percent sign and put the number over 100.

Example 1

Change 24% to a fraction.

Solution

Drop the percent symbol and put 24 over 100.

24%=24100  Percent: Parts per hundred.=625  Reduce.

Hence, 24% = 6/25.

Exercise

Change 36% to a fraction reduced to lowest terms.

Answer

9/25

Example 2

Change 1427% to a fraction.

Solution

Drop the percent symbol and put 1427 over 100.

1427%=1427100  Percent: Parts per hundred.=1007100  Mixed to improper fraction.=10071100  Invert and multiply.=10071100  Cancel.=17

Hence, 1427%=1/7.

Exercise

Change 1119% to a fraction reduced to lowest terms.

Answer

1/9

Example 3

Change 28.4% to a fraction.

Solution

Drop the percent symbol and put 28.4 over 100.

28.4%=28.4100  Percent: Parts per hundred.=28.41010010  Multiply numerator and denominator by 10.=2841000  Multiplying by 10 moves decimal point one place right.=7142504  Factor.=71250  Cancel common factor.

Exercise

Change 87.5% to a fraction reduced to lowest terms.

Answer

7/8

Changing a Percent to a Decimal

To change a percent to a decimal, we need only remember that percent means “parts per hundred.”

Example 4

Change 23.25% to a decimal.

Solution

Drop the percent symbol and put 23.25 over 100.

23.25%=23.25100  Percent: Parts per hundred.=0.2325  Dividing by 100 moves decimal point 2 places left.

Therefore, 23.25% = 0.2325.

Exercise

Change 2.4% to a decimal.

Answer

0.024

This last example motivates the following simple rule.

Percent to a Decimal

To change a percent to a decimal, drop the percent symbol and move the decimal point two places to the left.

Example 5

Change 512% to a decimal.

Solution

Note that 1/2=0.5, then move the decimal 2 places to the left.

512%=5.5% 1/2=0.5.=0.055  Drop % symbol. Move decimal point 2 places left.=0.055

Thus, 512%=0.055.

Exercise 8.1.1

Change 634% to a decimal.

Answer

0.0675

Changing a Decimal to a Percent

Changing a decimal to a percent is the exact opposite of changing a percent to a decimal. In the latter case, we drop the percent symbol and move the decimal point 2 places to the left. The following rule does just the opposite.

Decimal to a Percent

To change a decimal to a percent, move the decimal point two places to the right and add a percent symbol.

Example 6

Change 0.0725 to a percent.

Solution

Move the decimal point two places to the right and add a percent symbol.

0.0725=007.25%=7.25%

Exercise

Change to 0.0375 to a percent.

Answer

3.75%

Example 7

Change 1.025 to a percent.

Solution

Move the decimal point two places to the right and add a percent symbol.

1.025=102.5%=102.5%

Exercise

Change 0.525 to a percent.

Answer

52.5%

Changing a Fraction to a Percent

One way to proceed is to first change the fraction to a decimal, then change the resulting decimal to a percent.

Fractions to Percents: Technique #1

To change a fraction to a percent, follow these steps:

  1. Divide numerator by the denominator to change the fraction to a decimal.
  2. Move the decimal point in the result two places to the right and append a percent symbol.

Example 8

Use Technique #1 to change 5/8 to a percent.

Solution

Change 5/8 to a decimal, then change the decimal to a percent.

To change 5/8 to a decimal, divide 5 by 8. Since the denominator is a product of twos, the decimal should terminate.

Screen Shot 2019-09-20 at 4.13.24 PM.png

To change 0.625 to a percent, move the decimal point 2 places to the right and append a percent symbol.

0.625 = 0 62.5% = 62.5%

Exercise

Change 5/16 to a percent.

Answer

31.35%

A second technique is to create an equivalent fraction with a denominator of 100.

Fractions to Percents: Technique #2

To change a fraction to a percent, create an equivalent fraction with a denominator of 100.

Example 9

Use Technique #2 to change 5/8 to a percent.

Solution

Create an equivalent fraction for 5/8 with a denominator of 100.

58=x100

Solve this proportion for x.

8x=500  Cross multiply.8x8=5008  Divide both sides by 8.x=1252  Reduce: Divide numerator and denominator by 4.x=62.5  Divide.

Thus,

58=62.5100=62.5%.

Alternate Ending

We could also change 125/2 to a mixed fraction; i.e., 125/2 = 62 1 2 . Then,

58=6212100=6212%.

Same answer.

Exercise

Change 4/9 to a percent.

Answer

4449%

Sometimes we will be content with an approximation.

Example 10

Change 4/13 to a percent. Round your answer to the nearest tenth of a percent.

Solution

We will use Technique #1.

To change 4/13 to a decimal, divide 4 by 13. Since the denominator has factors other than 2’s and 5’s, the decimal will repeat. However, we intend to round to the nearest tenth of a percent, so we will carry the division to four decimal places only. (Four places are necessary because we will be moving the decimal point two places to the right.)

Screen Shot 2019-09-20 at 4.13.24 PM.png

To change the decimal to a percent, move the decimal point two places to the right.

0.3076 ≈ 0 30.76% ≈ 30.76%

To round to the nearest tenth of a percent, identify the rounding and test digits.

Screen Shot 2019-09-20 at 4.23.25 PM.png

Because the test digit is greater than or equal to 5, add 1 to the rounding digit and truncate. Thus,

0.03076 ≈ 30.8%.

Exercise

Change 4/17 to a percent. Round your answer to the nearest tenth of a percent.

Answer

23.5%


8.1: Percent is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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