The original number of girls was 20% greater than 15 (the number of boys) giving 15+(20/100)*15 = <<15+(20/100)*15=18>>18
After the admission of transfer students, the number of girls doubled to become 18*2 = <<18*2=36>>36
In addition to the 15 boys, there are now 15+36 = <<15+36=51>>51 students in the class
#### 51