Three vans are used to carry 45 students in the applied probability class to participate in a state competition on random phenomena. The first van carried 12 students, the second van carried 15 students, and the third van carried 18 students. Upon their arriving at the place where the competition was taking place, one student was randomly selected from the entire group to receive a gift certificate for a free lunch at a local restaurant. What is the expected number of students in the van that carried the selected student?