Formula for percentage Increase and corresponding percentage decrease (and viceversa)

Hello,

Looking for a formula that would do the following:

I have two percentages. I would like the percentages to talk to one another. E.G. An increase in one percentage would result in a proportional decrease in the other percentage.

E.G. One percentage is 10%. If it increases to 15%, then the corresponding category should decrease from its current level of 10%, to 5%.

Thanks!

-J