I'm afraid you are mistaken about the way interest is charged on credit cards. The method most credit card companies use is called the Average Daily Balance(ADB) Method.
I'll try to explain how it works:
As most of us know, with revolving credit, interest is charged only if you don't clear off the entire balance within a month or by the end of the statement. Once you carry over a balance to the next month's statement, it start attracting interest.
Let's say you spent Rs.10,000 on your card & paid only Rs.2000 by the end of the statement. Then interest will be charged, but not exactly for the remaining Rs.8000. Let's say you use your card after another 10days & charge another Rs.5000 & then again after another 10days & charged another Rs.5000, then the average balance is calculated for the month, which will be the average of:
Rs. 8000 for the first 10days Rs. 13,000 for the second 10days Rs. 18,000 for the last 10days of the month
which is Rs. 13,000.
So, the interest rate of 2.25 to 3% will be applied to the construed balance of Rs.13,000.
As you can see, this will work out to be costlier than what you explained :)
