Percentages and Percent Change

A percentage is a measurement of the amount of something given. If everything is given, you have 100% of that thing. If nothing is given you have 0% of that thing. If you have half of that thing, you have 50% and so on.
A percentage can be calculated by dividing the amount given by the total amount and then multiplying by 100. See this in the following example.
Example:
Suppose you only have 4 beers left in your 12 pack. Then you have



left to get drunk. Simplifying, you find that you only have 33.3% of the 12 pack.

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If you are given two numbers or values represented by , you can calculate the percent change with the following formula:


Example
If you drink 2 of your 4 beers, you can calculate the percent change by letting

. You find the percent change to be


The negative sign indicates that you decreased the amount (by drinking them). A positive percent change would indicate an increase in the amount of beers.

Other Examples
(Other examples) Profit, population growth/decline, cell growth/decline.