You should make clear the difference between arrangement and combination. By arrangement, I mean taking out a specified number of elements from a given number of elements and sorting them (not only taking out the specified elements, but also sorting them). Combination means that only a specified number of elements are taken out from a given number of elements, regardless of sorting (only taking out).
When arranging, combining and mixing, take a topic as an example: how many five-digit odd numbers can a non-complex number of 0, 1, 2, 3, 4 and 5 form? Because the last position and the first position have special requirements, priority should be given to preventing unqualified elements from occupying these two positions. The last number * * has C 13, then the first number * * has c, the last line * * *, and there are A34 in other positions, so these numbers should be multiplied by the principle of step-by-step counting (according to 1, this event will only happen if they appear at the same time).