I have a problem: I want to receive a sum but, to receive that sum, I have to pay a percentual fee on this transaction.
For example: if I want to receive 20 and fee is 10%, I can't require 20 + 10% (so 22) because I would receive 19,80 (22 - 10%).
So, what is the correct formula to calculate the sum I have to require to receive an import?
You want to have a net value of $\$20$. And $10\%$ of the total payment is the fee. Therefore $90\%$ is equivalent to $\$20$. Now you calculate the equivalent to $100\%$
Thus the total amount, which has to be payed is $20\cdot \frac{100\%}{90\%}=22\frac{2}{9}\approx 22.22$
And the fee is $\$22.22-\$20=\$2.22$