A' = A, B' = B- 1, C' = C-2, D' = D-3, then, a', b', c', d', all > = 1

a'+b'+c'+d'= 10

At this time, the partition method is used, and C9(3)=84.

This method can be used for this kind of quota allocation problem in the future. The requirement is that each class is greater than or equal to 1