Let's call the starting value 'x'
Let's call the final value 'y'
x * 1.15 will give you the 115% (your 115 value)
(x * 1.15)*(0.85) will give you 15% off the value of 115% (your 97.75 value)
So, you can write the expression as:
1.003x = y
What you seem to be asking for is bizarre, as you would simply be multiplying by 1.