The percent change is given by ...
... (percent change) = (new amount - old amount)/(old amount) × 100%
This can be rearranged to give a formula for the new amount. First, we'll rewrite it to a slightly different form.
... (percent change) = ((new amount)/(old amount) -1) × 100%
... (percent change)/100% = (new amount)/(old amount) -1 . . . . divide by 100%
... (percent change)/100% + 1 = (new amount)/(old amount) . . . add 1
... (old amount) × ((percent change)/100%) +1) = new amount . . . . multiply by old amount
We can now use this formula to find the new amount in each case.
13. 25 × (300%/100% +1) = 25 × 4 = 100 . . . . dollars
14. 160 × (-20%/100% +1) = 160 × 0.8 = 128 . . . . bananas
15. 56 × (-75%/100% +1) = 56 × .25 = 14 . . . . books
16. 52 × (25%/100% +1) = 52 × 1.25 = 65 . . . . companies
17. 12000 × (5%/100% +1) = 12000 × 1.05 = 12,600 . . . . miles
18. 710 × (-10%/100% +1) = 710 × 0.90 = 639 . . . . points
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Considering the above formula for percent change (or its "slightly different form"), you may want to reconsider your answers for problems 7–12.