MathBench > Miscellaneous

Tricks with Division

Section 3: Percent Change

Measuring how things change

Finally, there is a measurement that we often make when we’re comparing the results of some sort of experiment. We want to know how much things changed as a result of whatever we did – if I add fertilizer, how much taller does my plant grow? If I feed my mouse less, how much longer does he live? And so on. Or, we might just want to know how things change over time – such as, how did the price of gasoline change from 1998 to 2008?

gas stationTo look at how things change, we use percent change. Here’s how it works. Let’s say I want to know the percent change in the price of a gallon of gas over the course of the last decade.

First I calculate the actual change, which is

$4.20 - $1.40, or $2.80

Then I compare the change in price to the baseline price – that is, the price at the beginning of the timeperiod. So my ratio looks like this:

change in price : baseline price = $2.80 / $1.40

= 200%

Likewise I could compare the percent change in fat in the Old-Fashioned Pound Cake vs. the Lowfat Pound Cake.

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I think I have the answer: -19%


The minus sign tells me that the amount went down compared to the baseline.