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

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.