I have a number and the percentage this number represents - how do I calculate what the full value is? 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?
 A: 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! 
