Answer: 25% increase
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Work Shown:
A = old price = 320
B = new price = 400
C = percent change
C = [ (B-A)/A ] * 100%
C = [ (400-320)/320 ] * 100%
C = (80/320)*100%
C = 0.25*100%
C = 25%
The positive C value indicates a percent increase.
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A slightly alternative method:
A = old price = 320
B = new price = 400
C = change in price
C = B-A
C = 400-320
C = 80 is the price increase
D = percent change
D = (C/A)*100%
D = (80/320)*100%
D = 0.25*100%
D = 25%
We get the same result as before.