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This page says- absolute change = new quantity – old quantity.

For ex- Example 1: On September 20, a gallon of gas at my usual gas station cost $1.83. On September 30, I noticed that the price had gone down to $1.65 per gallon. The absolute change in the price of gas between September 20 and September 30 is

              1.65 – 1.83 = - 0.18 dollars per gallon

We say that the price of gas decreased by $0.18 (or 18 cents).
And this page also says

Percentage change = absolute change/original quantity*100

Example 1 revisited: On September 20, a gallon of gas at my usual gas station cost $1.83. On September 30, I noticed that the price had gone down to $1.65 per gallon.

The percentage change in the price of gas between September 20 and September 30 is

(1.65 – 1.83) 100 = - 0.18(100) = -18% per gallon

We say that the price of gas decreased by 18% per gallon.

But from the formula of percentage change I can see that they are not dividing it by origianl quantity. Is this right?

Please help me figure out.

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    $\begingroup$ Yes, it's a typo. You'll notice in example 2 they do it correctly. $\endgroup$ – Bangkockney Sep 7 '15 at 4:42
  • $\begingroup$ They also should not have said per gallon. $\endgroup$ – André Nicolas Sep 7 '15 at 4:43
  • $\begingroup$ thanks... It was confusing me. $\endgroup$ – RajSharma Sep 7 '15 at 4:44
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It should be $\frac {1.65-1.83}{1.83}100\%\approx 9.8\%$ The definition you have quoted is clear.

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