Francis has a credit card that uses the previous balance method. the opening
balance of one of his 30 day billing cycles was 80, but this was his balance
for only the first 15 days of the billing cycle. he then made a purchase that
increased his balance to $3600, and his balance stayed this amount for the
remainder of the billing cycle. if his credit card's apris 21%, how much was
francis charged in interest for the billing cycle?