I don't know a good way to word the question, so here's an example:

I have the number 30. I want to add a number to it such that the number I added is 25% of the new number. In this case, I need to add 10, making the total 40 and 10 is 25% of 40.

Those numbers are easy but I need a general rule to find how to get 10.

I'm stuck because I'm missing TWO pieces of information: how much to add, and what the total will be. This is what I have:

Where N is current number, x is number to add,
t is the new total, and P is the desired percentage...
N + x = t
x = t * P
therefore...
N + x = x/P

I haven't had to do algebra in like 8 years and I'm honestly upset with myself that I can't figure this out. I can't figure out how to isolate x because it's only in one of the two terms on the left side and it's over P on the right side, so continuing to simplify just gives me

PN + Px = x
PN = x - Px

and now I'm stuck again.

Edit: FACTORING!! I completely forgot about that. I can't believe how far I've declined since high school; factoring used to be near the bottom barrel.

I've finally gotten the solution:

PN = x * (1 - P)
x = (P * N) / (1 - P)

That was more painful than it should have been.