Estimating the amount of interest if it is mixed with the original money
Question

                My father has left for me an amount of money in an interest-based bank. I have owned the account for 21 years and I do not know if there is added interest on the original money or not. I know that my parents have added more money into this account for me, which reduces my doubt about how much interest is added. I have closed this account and taken the money, I do not know how much interest is mixed with this money, and I would not know even if I still had the account. What should I do?.
Praise be to Allah.Money deposited in the bank 
    is in fact a loan, even if it is in a savings account, and the interest paid 
    on them is haraam riba which the texts forbid, and issue a stern warning to 
    the one who consumes it. 
In a statement of the 
    Islamic Fiqh Council belonging to the Organization of the Islamic 
    Conference, which was held in Jeddah in 1406 AH, it says the following: 
Everything that comes by 
    way of riba and interest is wealth that is forbidden according to sharee’ah, 
    and it is not permissible for the Muslim – the one who deposited the money – 
    to benefit from it for himself or for any of his dependents in any of their 
    affairs. He must dispose of it by spending it on the public interests of the 
    Muslims, such as schools, hospitals and the like. This does not come under 
    the heading of charity, rather it is purifying his wealth of haraam 
    elements. 
See: Majallat al-Majma’ 
    (9/1/667). 
Based on that, you should 
    do whatever you can to find out the amount of riba, by referring to the bank 
    system and the way in which such interest payments are made, and ask your 
    parents about the original amount that they put in this savings account, and 
    the money that they deposited more recently. If you find out the precise 
    amount of interest, then you have to dispose of it by spending it on the 
    public interests of the Muslims. Otherwise you should work out what you 
    think is most likely, and consult your parents with regard to that. 
And Allaah knows best.