As a student, Ivan was forced to carry a balance on his credit card because his funds were always tight

As a student, Ivan was forced to carry a balance on his credit card because his funds were always tight.  However, upon getting a job, he made it a point to pay extra on his card each month.  Finally, he paid his bill in full last month and did not charge any more items.  When he got his statement this month, he was expecting to see a zero balance, but instead was shocked to see that he owed $10.50.  It is likely that his credit card uses
 

Options: 

1. a single-cycle method to calculate the balance on which interest charges are applied