I have two known values : A sum and a percentage.
I know that the sum is $16 000$, and this is $28\%$ of unknown value. How do I calculate what this value is? Like; $16000$ is $28\%$ of $x$?
Doing $16000 \cdot 1.28$ gives me $20480$. But that's wrong?
Let our unknown quantity be $q$. Then $28\%$ of $q$ is $16000$. In symbols, $$(0.28)q=16000.$$ Divide both sides of the above equation by $0.28$. We get $$q=\frac{16000}{0.28}.$$ Now it is probably best to use a calculator. We get $q\approx 57142.86$.
If you prefer, you can think of $28\%$ as $28$ "per centum," meaning out of $100$. So $28\%$ means $\frac{28}{100}$. Thus we could write our equation as $$\frac{28}{100}q=16000.$$ Multiply both sides by $100$, and then divide both sides by $28$. We get $$q=\frac{16000\times 100}{28}.$$
Remark: It is always possible to make mistakes in this sort of calculation, a little slip in algebra, or a calculator keying error. So it is a very good idea to check whether the number you obtained is right. In this case, we do that by finding $28\%$ of our supposed answer of $57142.86$.
Added: OP has indicated that the real problem is as follows. Tax is $28\%$. a project pays $16000$ after tax. What is the before tax income from the project?
Let $b$ be the before tax income. Then $b$ minus $28\%$ of $b$ is $16000$. That means that $72\%$ of $b$ is $16000$. Using exactly the same reasoning as before, we have $$(0.72)b=16000.$$ Using exactly the same procedure as before, we conclude that $$b=\frac{16000}{0.72}.$$ The calculator gives $b\approx 22222.22$. Cute!