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 first 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?