Hey there,
Okay so for inverse equations I use the method in which you switch the variables and solve for y.
So the equation y = 100 - x would look like:
Now we solve for y in this equation:
Since the y is negative and we are solving for a positive we divide by -1 on both sides:
So the inverse of y = 100 - x would have to be -x + 100 = y.
Hope I helped,
Amna