The members of the city cultural center have decided to put on a play once a night for a week. Their auditorium holds 600 people. By selling​ tickets, the members would like to raise ​$3,900 every night to cover all expenses. Let d represent the number of adult tickets sold at ​$8.50. Let s represent the number of student tickets sold at ​$5.50 each. If all 600 seats are filled for a​ performance, how many of each type of ticket must have been sold for the members to raise exactly ​$3,900​? At one performance there were three times as many student tickets sold as adult tickets. If there were 480 tickets sold at that​ performance, how much below the goal of ​$3,900 did ticket sales​ fall?