If you start with some stuff, then later you have more or less stuff, what percentage did the amount of stuff you have change?
Comparing two numbers by describing its “percentage increase/decrease/change” is a common math skill.
Now keep in mind that to an English student the words “increase” and “decrease” are obviously opposite, but to a Math student the words can mean the same thing – a negative increase is equivalent to a positive decrease.
If you start with 40 “stuff” and end with 20 “stuff” would you describe it as a 50% decrease or a -50% increase?
http://www.ehow.com/how_4449676_calculate-percentage-increase.html
http://www.marshu.com/articles/calculate-percentage-increase-decrease-percent-calculator.php
http://www.tellmehowto.net/howto/calculate_percentage_change_6409
http://www.ask.com/questions-about/Calculate-Percentage-Change
