A school theater program is producing a show and needs to raise at least $15,000. The school's auditorium has 500 seats. Historically, the program sells twice as many adult tickets as child tickets. If the price of a child ticket (c) is 20% less than the price of an adult ticket (a), what is the minimum price the program must charge for each type of ticket to reach its revenue goal? (Round your answers to the nearest cent.)