Math problem: the total number of different distribution methods that require at least one in each class when five outstanding graduates give lectures in three classes of their alma mater.
3+ 1+ 1 allocation mode
C(5，3) A(3，3)
= 10×6
=60 (species)
2+2+ 1
C(5 1)C(3 1)C(4，2) C(2，2)
=5×3×6× 1
=90 (species)
Therefore, there is always a method of distribution.
60+90= 150 (species)