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How do I calculate percent change

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
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by boyerml » Tue Nov 03, 2009 8:02 am
In the phrase

"has increased from 12.1 percent to 20.4 percent"

I would like to state that "disease" has increased by a certain percent. The difference here is 8.3%. So I need to know how I word this AND calculate it.

If I divide 12.1% by 20.4% I have a change (increase) of 59.3%

If I divide 8.3% by 12.1% my change is 68.6%.

Which is the most correct way to present the increase?

Which is the best wording to express that change:
"disease" has increased by xx.x%
or
"disease" has increased xx.x%

Thanks
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by regor60 » Thu Nov 05, 2009 9:46 am
12.1/20.4 is a meaningless comparison since the base is the new base and you lose the "from" anchor.

You could say "the infected population has increased by 68.6%, or by 8.3% as a percentage of the entire population"
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by mjsobo » Thu Nov 05, 2009 1:08 pm
the proper way to calculate percentage change is (d2-d1)/d1, where d2 is your new number. in simple terms, it is the delta (the change) divided by the original number. so in this case it is 8.3/12.1

And i agree with the response below, would be good to say that "the infected population has increased by 68%".

good luck!
Martin
GMAT Instructor with Grockit
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thank you

by boyerml » Thu Nov 05, 2009 4:03 pm
Thanks to both of you. That always seemed to make more sense to me, but I was never sure and didn't want to over state the increase and lose credibility for the article. Thanks again

Michele
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