percentage problems & guide

link is:http://aptitude9.com/short-cut-methods-quantitative-aptitude/
linlk is:http://mathcentral.uregina.ca/QQ/database/QQ.09.06/h/anna2.html


I would calculate the difference as the new value minus the old value. If this difference is positive there has been an increase and if it is negative there has been a decrease. You then want to state this change as a percentage but the question is a percentage of what? Not everyone does the same thing. I think you should report the percentage change as a percentage of the old value so the expression would be

(new value - old value)/old value ×100

What is important is how you state the result so it is clear what you did.
Suppose the 2005 value was 400 and the 2005 value was 500 then I would calculate

(500 - 400)/400 ×100 = 25%

and say "In 2006 we had an increase of 25% of the 2005 value".
Suppose the 2006 value was 500 and the 2007 value was 300 then I would calculate

(300 - 500)/500 ×100 = -40%

and say "In 2007 we had an decrease of 40% of the 2006 value".