Percent Introduced
The word percent simply means “out of
100.” The language of percent is used by everyone frequently
as a convenient and intuitive was to avoid using the terminology
of numerical fractions. What we will try to do in this section of
notes is describe very precisely how you can use the terminology
of percent to clearly and accurately communicate numerical
information.
Familiar examples of the use of the word percent are:
 percents which express a part or fraction of a whole.
 percents which express a rate of change in some quantity.
In a way, this is not really different than percents
which express a fraction of a whole because the percent
as a rate of change is expressing the amount of change as
a fraction of the original quantity.
Doing Arithmetic With Percents
The thing to remember is that percents represent the numerator
of a fraction that always has a denominator of 100. So, when we
say, for example, 25%, we are really speaking of the fraction or 0.25 in decimal form.
So conversion between a percent, an actual fraction with a
numerator and denominator, and a decimal fraction is very easy to
do.
If you start with the percent then
 to get the fractional equivalent, just write a fraction
with the percent in the numerator and a denominator of
100. For example
 to get a decimal fraction equivalent, just divide the
percent by 100 (which is the same thing as moving the
decimal point two places leftwards). For example
 If you start with a proper fraction or a decimal
fraction, just multiply by 100. For example
 This rule also works if the starting fraction has a
denominator which is not equal to 100. For example
 It may be that this conversion results in digits to the
right of the decimal point. For example
 rounded to two decimal places. How many decimal places
should be retained in the final percent that is
calculated depends on the situation.
Finally, notice that percents can be (and often are) larger
than 100. Such percents just correspond to fractions and decimal
numbers which are bigger than 1. For example
The meaning of percents bigger than 100% will depend on the
situation. When a percent is expressing a part or fraction of a
whole, it makes no sense to have a value bigger than 100% (since
then the part would be more than the total stuff that it is part
of). However, if the percent is referring to a rate of increase,
values bigger than 100% just mean that the quantity has more than
doubled in size.
(People occasionally speak of negative percents to indicate
rates of decrease. We will need to look at expressing rates of
decrease very carefully, but probably the complicated notion of
negative percents should be avoided to avoid confusion.)
