Annette has a credit card that uses the previous balance method. The
opening balance of one of her 30-day billing cycles was $2990, but that was
her balance for only the first 7 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 31%, which of these expressions could be used to calculate the
amount Annette was charged in interest for the billing cycle?

Question