At the start of january, you have a credit card with a balance of $498, a credit limit of $900, and an interest rate of 15%. you make monthly payments of $35. at the end of which month will your balance be below the target debt ratio.

There may exist a formula for this problem, but it might be simpler just do the month to month calculations, since the maximum number of months is four, and minimum 2.

Target debt is $900/2=450 or less.

At the end of January:
Debt carried over=$498
 add interest = $498*0.15/12)=$6.30;
total debt=498+6.30=504.30
debt after payment = 504.30-35=$469.22

continue as above until debt after payment is at or below $450.