The drama club is selling tickets to their play to raise money for the show's expenses. Each student ticket sells for $6 and each adult ticket sells for $8. The auditorium can hold a maximum of 100 people. The drama club must make at least $680 from ticket sales to cover the show's costs. If 55 student tickets were sold, determine the minimum number of adult tickets that the drama club must sell in order to meet the show's expenses.