Question 903860
x - 15% of x = x - 0.15x = 0.85x


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Old Salary: x dollars


New Salary: 0.85x dollars


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Now we want to go from 0.85x to x dollars again. 


Difference: x - 0.85x = 0.15x


Divide this difference by the amount you are increasing from (which is 0.85x)


0.15x/0.85x = 0.17647


0.17647 ----> 0.17647*100 = 17.647%


So you need to increase his salary by roughly 17.647% to get back to the original salary.


Side Note: a common mistake is to think that you increase it by 15% to get back to the original, but taking 15% of a smaller salary leads to a smaller increase. So that's why the percentage needs to be bumped up to get it back to the original.