What Formula should I use to calculate interest? Interest is 15% APR, monthly payment $400, new monthly charges $300?

Part 2: A person holds a credit card balance and continues to make charges as well as payments each month. For each month's payment, the person has 2 options: either makes a regular $400 payment each month OR pay the minimum payment (a deceptively attractive option) required each month. Explore the consequences of each option by completing each table and summarizing the results. New Credit Card Charges $300.00 Previous Balance $2,400.00 Interest Payment # 1 2 New Balance Payment $400.00 2 3 4 5 6 7 8 9 10 11 12 Month January February March April May June July August September October November December January February March April May June July August September October November December 13 14 15 16 17 18 19 20 21 22 23 24