In fact, the problem is that there are two people in 1 heap, and there are two people in 1 heap. Two out of four people should be selected to form 1 pile, and the other two piles should be ignored, so C42*A33=36 (species).

Permutation and combination is the most basic concept of combinatorics. The so-called arrangement refers to taking out a specified number of elements from a given number of elements for sorting. Combination refers to taking out only a specified number of elements from a given number of elements, regardless of sorting.

The central problem of permutation and combination is to study the total number of possible situations in a given permutation and combination. Permutation and combination are closely related to classical probability theory.

difficulty

1. Abstracting several concrete mathematical models from different practical problems requires strong abstract thinking ability.

2. Restrictive conditions are sometimes obscure, which requires us to accurately understand the key words (especially logical related words and quantifiers) in the question.

3. The calculation method is simple and has little connection with the old knowledge, but a lot of thinking is needed when choosing the correct and reasonable calculation scheme.

4, the calculation scheme is correct, often can't use intuitive method to test, requires us to understand the concept, principle, and have strong analytical ability.

Refer to the above content: Baidu Encyclopedia-permutation and combination