Percentage Target (Projection)Date: 07/24/2003 at 05:34:41 From: Sol Subject: Percentage target (projection) Hi there, I am trying to work out percentage difference between a given figure and a projected figure in the future. For example, say in 2000 I have a percentage of 90.4%, and by 2003 I need to work out a 10% decrease in my original figure. I work out a 10% decrease of 90.4% to be 80.4%, but I know this is not correct. Thank you. Date: 07/25/2003 at 11:26:51 From: Doctor Greenie Subject: Re: Percentage target (projection) Hi, Sol - There are two distinct ways of thinking of a 10% decrease. Because of the word "decrease" (I suppose), most people think of a 10% decrease as the original amount minus 10% of the original amount. However, in most cases, the arithmetic required to work with percentages is easier if, instead, you think of a 10% decrease as meaning that 90% is left. So if you are starting with a figure of 90.4%, then a 10% decrease from that means you have 90% of the 90.4% left: (.90)*(.904) = (.8136) = 81.36% You noted that you didn't think 80.4% was the correct figure; you are right. It is not the right figure because you subtracted 10% of the "original total" instead of 10% of the 90.4% you started with. The alternative to the above calculation is to subtract 10% of the 90.4% from the 90.4% you started with: .904 - (.10)*(.904) = .904 - .0904 = .8136 = 81.36% I hope this helps. Please write back if you have any more questions about this. - Doctor Greenie, The Math Forum http://mathforum.org/dr.math/ |
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