This is a bit of a strange question, I came up with a simple analogy.
Say I have test results, for a group of students. And percentage of students passed is $p$, failed is $f$.
I want to calculate the value of $p$ from $f$, bear in mind they are both percentages. Say $p$ is 25%, then $f$ would be 75%. How could I calculate this with $p$ as any real number?
EDIT: $100 - p = f$
sigh
Percentages add up to 100, so 100 - p = f