I am going to deposit 261.33€ on an account each month.
Each month, 0.1% of the total value of the account is debited. So for the first month with the account starting at 0, it would be 261.33 * 0.1% = 0.2613€.
After a year, 3200€ have been deposited. What is the formula to figure out how much has been debited by these monthly 0.1% ?
The formula is $$3135.96-\Bigg(261.33\sum_{i=1}^{12}0.999^i\Bigg)$$
Where 3135.96€ is the actual amount of 261.33€ times 12. It gives approximately 20.31€ if you don't lose precision (okay cut precision a little to do it on a basic phone calculator) . There are easier ways to approximate financial math as well.