John accepted a reduction of 15% on his salary when his company was not doing well. now his company's financial position has improved and his boss wants to restore his original salary. by what percentage must his reduced salary be increased?

To reverse a percentage decrease you divide it by the decrease (+ 100%)
For example we will pick the number, 100, which is decreased by 15% to 85.

To make 85 back to 100 we divide it by the decrease (1-0.15):
85 / 0.85 = 100

To find out how much 85 increased to get back to 100, we do:
15 / 85 = 0.1765 = %17.65

15 is the reduction/difference, and 85 is the with reduction total.

Because percentages stay the same, this is applicable to any numbers, from this, we know that whenever something is reduced by 15% - when restored to it's original is increased by %17.65

The answer is %17.65