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When someone uses the phrase "percent increase" what does that mean?

For example, if something took $4$ seconds before and now it takes $1$ second, would that be a $400$% increase?

hbhakhra
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  • I'm new to this stack exchange; care to clarify the down vote on the question? – hbhakhra Feb 21 '14 at 00:37
  • I can't see a reason to down vote your question. I wouldn't worry about it. – 2'5 9'2 Feb 21 '14 at 00:39
  • If it goes from 4 to 1 it would be a $75%$ decrease in time, or a $300%$ increase in speed. Usually when people talk of percent increase or decrease they are up to no good and often they are incorrect mathematically. – Ross Millikan Feb 21 '14 at 00:46

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$A$ is a $B\%$ increase over $C$ iff $100\times(A/C-1)=B$ where the minus one is because of the "increase" part (and the 100 is because of the "percent" part).

That is, 4 is a 300% increase over 1. A 0% increase over 1 would just be 1, while a 100% increase would be 2.

jazzwhiz
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