Suppose that four teachers, A, B, C and D, teach in 1, 2, 3 and 4 classes, then during the examination,

Each teacher invigilates a class, which is a combination, with

4×3×2× 1=24 roads,

The situation of teachers in their own classes in these 24 combinations is:

There is only one teacher in our class: suppose A is in 1 class, then Class 2 has C and D invigilators, and Class 3 and Class 4 each have one invigilator.

The total is 4x2 = 8.

There are two teachers in our class: suppose A and B are in our class, then there is only one case, C and D are not in our class.

Then consider which two teachers have: 4×3/2 or 3+2+ 1=6 (permutation and combination) in their own class.

There are three teachers in our class: the fourth teacher must be in our class at this time, so it is 1.

Above, then * * * has the invigilation method.

24-8-6- 1=9 (species)