1.总数：C(18,54)(1)班有一个：C(1,3)*C(17,51)(1)班有两个：C(2,3)*C(16,51)(1)班有三个：C(3,3)*C(15,51)P=(C(1,3)*C(17,51)+C(2,3)*C(16,51)+C(3,3)*C(15,51))/C(18,54)2.总数：C(18,54)每班都有：(C(1,3))^18，即3^18P=3^18/C(18,54)
