Shirley has a credit card that uses the previous balance method. the opening balance of one of her 30-day billing cycles was $2830, but that was her balance for only the firet 2 days of the billing cycle, because she then paid off her entire balance and didn't make any new purchases. if her credit card's apr is 19%, which of these expressions could be used to calculate the amount shirley was charged in interest for the billing cycle?