How can the amount of usurious interest be determined from money mixed with usury in a single savings account where deposits and withdrawals of interest have repeatedly occurred, and what is the ruling on Zakat paid on the total amount?

It is forbidden to deposit money in usurious banks, except in cases of extreme necessity where no Islamic bank is available, such as fear of theft or damage. Whoever has done so must repent, withdraw their money, and dispose of the interest by giving it to charitable causes. The interest is calculated by knowing the principal and what has been added to it, or by requesting an account statement, or by strong presumption with caution. Zakat is obligatory on the principal amount deposited, but it is not obligatory on the usurious interest because it is ill-gotten wealth that must be disposed of and is not counted towards Zakat.