so, look at the amounts, including daily interest. The 1st day's $200 was charged interest 21 times, the 2nd day's 20 times, etc. This adds up to
200(1+r)^21 + 200(1+r)^20 + ... + 200(1+r)^1
where r = 0.28/365
This is just a GP with the last term (200) excluded, where
a = 200
r = (1+0.28/365)
You want S21-200
Just plug your numbers into the GP sum formula which you know.
While she was travelling, Zaina took advantage of the convenience of cash withdrawals on her credit card since her Canadian debit card wasn’t accepted in the country she was in. According to her travel budget she withdrew $200 every day for food, activities and shopping for 21 days.
When she got home, on the 21st day, she checked her credit card bill on-line and it showed that she had been charged interest already even though her payment wasn’t past due. It turns out that interest is compounded daily on cash withdrawals, beginning from the moment the cash is withdrawn. If the interest rate on cash withdrawals is 28%, what was her total bill when she got home?
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