>Hi everybody,
>
>If I have an item which cost $100 and the annual interest rate is 2% and the days of interest 10, how can I calculate the actual interest cost?
>
>Can you give me the exact formula, please?
>
>Thanks.

I believe, it depends on the method of calculation and definition of "annual interest rate", that partucular party is using.
If it is a credit card, and you are getting bill by ordinary mail, then usually description is printed there. Having description you may derive a formula.