Like Negative Percent? [Resolved]

Alright here goes, I've search this math forum for my answers but cant find it.

I need a formula Calculates the Value of X

100 * (0.01 to 0.99) = A
A * X = 100

I dont know if what I just typed made any sence but Basically I want to know how to find out a number before a percentage was taken away from it.

Example:
100 * 0.80 = 80 (80 percent)
BUT
80 * 1.20 != 100
SO
80 * X = 100

How do I find out for X?

I want to take a fairly random number Take a Percent away from it then later get the orginal number again.

I think I know what you mean

eg
Suppose a number X has had 20% taken away from it

Then you have 0.8 * X

Now suppose you know what the answer is but you want to find X again

Then you have to undo the effect of multiplying by 0.8 by dividing by 0.8

eg 2 A number has 16% taken away and is now 28. Find the original number

100 - 16 = 84

28/0.84 = 33 1/3

Thanks for the Reply, I cant beleive I didn't think of that.
Thanks.