Using Percents
Most students learn to use percents in middle school, but many people still have trouble interpreting information involving percents even as adults. Just what is a percent (%)?
The word percent comes from the Latin per centum meaning “out of one hundred,” so we can think of 22% as “22 out of 100.” Thus a percent is a symbol representing a ratio of the part — in this case 22 — to the whole — 100. A percent must be changed to a number (fraction or decimal) before we can compute with it. Hence 22% is the ratio 22 : 100, which gives:

=  .22 
Other examples:
percent notation  ratio notation  number notation 

30%  30 : 100  30 / 100 = 0.3 
8%  8 : 100  8 / 100 = 0.08 
63.7%  63.7 : 100  63.7 / 100 = 0.637 
100%  100 : 100  100 / 100 = 1 
212%  212 : 100  212 / 100 = 2.12 
5 3/4%  5 3/4 : 100 or 5.75 : 100  5.75 / 100 = 0.0575 
We will most often use the decimal form (in bold above) because it is easy to find on a calculator by dividing by 100. Let’s make some notes from these examples:
 The mechanics of changing a percent to a decimal number involves moving the decimal point two places to the left. The percent symbol can help us remember this:
 When we change percents smaller than 10 percent to numbers, we need to place holding 0s — as with 8% and 5 3/4%.
 100% is 1, reinforcing the idea that x% describes x’s part of “the whole.”
 Percents representing more than 1 are possible (212%) — they are used to talk about something growing really large rather than as “part out of the whole.”
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