嘻嘻，这是俺这周给大学生上课的内容 :)

初中生学过排列组合的定义没？

－－俺的学生好像以前不太会，可他们都会用计算器按 P(n,r),C(n,r). 所以肯定接触过点？

Anyway, for this one, if the kid knows C(n,r):
List all 12 marbles in a row by choosing 3 position for blue ones,then 4 positions for red ones, and the remaining for green. There are C(12,3)C(9,5) ways.
Then cut the row into three non-empty parts, in C(13,2) ways. So the answer is C(12,3)C(9,5)C(13,2).

其实这题有点模糊的地方。上面答案是假设the three rows are distinguishable. If they are not distinguishable, the answer is much harder.