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Whenever a price is changed, you can find the percent of increase or the percent of decrease by using the following formula:

$$\frac{\text{percent of change}}{100}=\frac{\text{change in price}}{\text{original price}}$$

To find the change in price, you calculate the difference between the original price and the new price.

Is the "percent of change" the change in price represented as a percent of the original price?

Does the proportion:

"percent of change" is to $100$ as change in price is to original price

make sense? Also, don't we lose the percent symbol if the original price is $100?

Since then

$$\text{percent of change}=\text{change in price}$$

So does "percent of change" now just become a portion of the original price?

If I replace the word "percent" in the above formula with "portions of 100" the whole thing makes a lot more sense, because of the literal meaning of "percent" being "for each 100" in my opinion.

Edit: the formula also makes no sense if the original "price" is zero.

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    $\begingroup$ Yes. You're right. $\endgroup$ – L__ Mar 7 '14 at 10:18
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Confusion arises from the use of units. Suppose we want the percentage change to be $a$ per cent. Let the change in price be $b$ dollars and the original price be $c$ dollars. Then we have $\frac {a}{100}=\frac bc$.

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