how to develope a interest calculation program

I would like to develop a program for calculating interest for loan account information(Example information) .Iwant how to write a program to get number of days used in particular amount in each month and how to calculate interest for quarterly basis.

example 19-04-2006 I am taking a loan Rs 100000/- at the rate of 12%

I was paid an amount Rs 3000/- as on 21-05-2006

I was paid an amount Rs 4500/- as on 28-06-2006

I was paid an amount Rs 2800/- as on 19-07-2006.

I would like to calculate interest from 19-04-2006 to 31-07-2006.

Please help me how to develope a program for above example

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