Answer: 80% increase
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Work Shown:
a = old value = 15
b = new value = 27
c = change in values
c = b-a
c = 27-15
c = 12
The positive c value tells us that we have an increase.
Then divide over the starting value a = 15
c/a = 12/15 = 0.80
Move the decimal point two spots to the right to go from 0.80 to 80%
Or multiply 0.80 with 100% to get 0.80*100% = 80%
We have a percent increase of 80%
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Check:
80% of 15 = 0.80*15 = 12
If the stock increases by 80%, then it has increased by $12 to go from $15 a share to 15+12 = 27 dollars a share. This confirms the answer.
A slight shortcut would be to say 1.80*15 = 27
The multiplier 1.80 represents an increase of 80% since 100% + 80% = 1 + 0.80 = 1.80