For some reason I'm having a brain freeze on this. I want to invert percentages. For example:

a = 1 : 17%
b = 2 : 33%
c = 3: 50%

Calculation is simply n/(a+b+c). But I want it inverted such that

a = 1 : 50%
b = 2 : 33%
c = 3: 17%

What's the formula I'm looking for?