If a pizza costed $7.50 in 2000. and a pizza costed $9.00 in 2015. what is the percent change in the cost?
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Work Shown:

A = starting value = 7.50

B = ending value = 9.00

C = percent change

C = 100*(B-A)/A

C = 100*(9.00 - 7.50)/7.50

C = 100*(1.50/7.50)

C = 100*0.2

C = 20%

The positive C value indicates we have a percent increase.

Take 20% of 7.50 and you'll get 1.50, which adds to 7.50 to get 9.00; this is a longer way of saying "the value has gone up by 20%"

Notice how I didnt involve the year values of 2000 and 2015 at all. They don't matter as you can use any two year values you want, and you would get the same answer. Those numbers were likely thrown in as a distraction.

both equations and proportions are better.