C2 1 times C4 2 equals 12.
C2 1 means to choose one from two classes first, and there are two choices.
C4 2 means to choose two teachers from four to teach the selected class.
Common methods of expanding data arrangement and combination
1, binding method
Questions with the characteristics of "together" and "adjacent" often appear in the combination of mathematical operations. At this time, we consider the binding method (also called packaging method by some teachers), that is, the elements that are "together" are "bound". The specific steps are: first sort within the binding, and then sort between the binding body and other elements.
2. Interpolation
In the problem of permutation and combination, several elements are often required to be "not together" and "not adjacent" when sorting. At this time, the interpolation method can be considered.