i'm assuming that your sister took 2000 out of her credit card and gave it to you as a loan.
then she had to pay off the 2000 at 22% annual interest rate per year.
the calculations would be as follows:
annual interest rate percent = 22%
annual interest rate = .22
monthly interest rate = .22/12 = .01833333333
in fractional form that would be equal to 22/1200.
assuming no other activity in the account, your balance at the end of each month would be as shown below:
month interest payment balance
0 0 0 $2,000.00
1 $36.67 200 $1,836.67
2 $33.67 100 $1,770.34
3 $32.46 100 $1,702.80
4 $31.22 100 $1,634.01
5 $29.96 100 $1,563.97
month 0 is when the loan is made.
month 1 is the end of the first month.
month 2 is the end of the second month.
etc.
you pay interest on the unpaid balance at the end of each month.
then you deduct the payment.
what is left is the remaining balance.
this same process occurs each month.
at the end of the fifth month, you have $1,563.97 remaining balance.
that's the amount of the loan that still remains to be paid.
that's the amount you owe your sister.
you already paid her a total of $600.00
the amount of the loan that was paid off was $2000.00 - $1,563.97 = $436.03
the difference between $600 and $436.03 was the interest that was paid off with the money you gave her.
pay her $1,563.97 and you should be square.