PL/SQL :: How To Calculate Interest Percentage Dynamically

look at these tables (Customer and Rates), having some sample data. I've created a function for returning the interest percentage for a particular customer based upon his/her investment start date and end date. I'm getting the desired result but I'm wondering if I need calculate the composite interest here...then how can I do that? Suppose a customer has invested some amount for 45 days, then I need to calculate the interest for initial 30 days as 5% and additional 15 days as 6% and I need to give the final amount. In this case how can I pull this task off? how can we do this interest calculation by an Oracle program?

Customer Table
CUSTOMER_ID     AMOUNT     START_DATE     END_DATE
1000             40000     15-JAN-13     15-FEB-13
1001             34000     15-DEC-12     15-FEB-13
1002             35678     15-NOV-12     15-FEB-13

Rates Table

RATE_PCT     LOW_TENURE     HIGH_TENURE
5                    0     30
6                    0     60
7                    0     90CREATE OR REPLACE FUNCTION
F_INTEREST_PCT (V_CUSTOMER_ID IN CUSTOMER.CUSTOMER_ID%TYPE)
  RETURN NUMBER AS V_RATE_PCT NUMBER;
 
[code]...