SQL & PL/SQL

Hi All,
Please 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? Please give me some ideas and correct me if I'm wrong. I just want to know how can we do this interest calculation by an Oracle program?

Regards
BS2012

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	90

CREATE OR REPLACE FUNCTION
F_INTEREST_PCT (V_CUSTOMER_ID IN CUSTOMER.CUSTOMER_ID%TYPE)
  RETURN NUMBER AS V_RATE_PCT NUMBER;
  BEGIN
    SELECT A.RATE_PCT INTO V_RATE_PCT
    FROM RATES A, CUSTOMER B WHERE (B.END_DATE - B.START_DATE) BETWEEN A.LOW_TENURE AND A.HIGH_TENURE
                               AND B.CUSTOMER_ID = V_CUSTOMER_ID;
    RETURN V_RATE_PCT;
  EXCEPTION
    WHEN NO_DATA_FOUND THEN
    ROLLBACK;
    DBMS_OUTPUT.PUT_LINE(SQLERRM); 
    WHEN OTHERS THEN
    ROLLBACK;
    DBMS_OUTPUT.PUT_LINE(SQLERRM);  
  END F_INTEREST_PCT;