The average weight of students in four classes A, B, C, D is 60 kg. The average weight of students of the respective classes A, B, C and D are 45 kg, 50 kg, 72 kg and 80 kg. If the average weight of the students of class A and B together is 48 kg and that of class B and C together is 60 kg, the ratio of the number of students in class A and class D is