When the bank pays you interest, the tax rate is 20%.
Tax is the money paid.
The interest rate is the percentage of your principal paid in a year or a month, usually within a year.
The pre-tax interest rate is the interest that the bank pays you in addition to the principal.
After-tax interest rate is the interest that the bank pays you after tax except the principal.
Y=X* interest rate * time *( 1- interest tax) (functional domain)
The sum of principal and interest = principal ... there is this law in the textbook.