Mathematics & Statistics Question!!(急~~)

A group must be more than 1 students.
There are 2 students in group 1,
then the sum of students in group 2 and group 3 is 12-2=10
2+8=3+7=4+6=5+5=6+4=7+3=8+2=10
There are 7 methods to seperate when there are 2 students in group 1.
There are 3 students in group 1,
then the sum of students in group 2 and 3 is 12-3=9
2+7=3+6=4+5=5+4=6+3=7+2=9
There are 6 methods to seperate when there are 3 students in group 2.
When there are 4,5,6,7,8 students in group 1:
There are 5,4,3,2,1 methods to seperate respectively.
When there are 9,10,11 students in group 1,
the number of students in group 2 or 3 or both of them must be 1 or 0.
Therefore there are totally 7+6+5+4+3+2+1=28 methods.