Mathematics high school probability problem! High reward! Seek an answer

The first question is the choice method of a pair of sisters, and the choice method of four people is different =(9*8/2! )/( 1 1* 10*9*8/4! )= 12/ 1 10=6/55

2\ ditto =19/(22 * 21* 20 *19/4! ) = 24/22 * 21* 20 =1/385, and then 1- 1/385=384/385

3\ Arrangement method, first exclude the tallest and shortest * * * There is an arrangement method 6! =720, then insert the highest 720*5 and the shortest 720*5*6.

A, the tallest and shortest sitting together, as two people insert a person =720*5, probability =720*5/720*5*6= 1/6.

B. If a person sits in the middle between the tallest and shortest, it is regarded as inserting two adjacent five vacancies into one, because there are 720*4*2 permutations without order. Probability =720*4*2/720*5*6=4/ 15.

4\ General arrangement method p=5! = 120

Every book is put together with books on the same subject =2! *3! *2=24, and the probability is 24/ 120 1/5.

Find the probability of putting chemistry books together first, and think that two chemistry books are 1. Probability =4! /5! = 1/5

The probability that chemistry books are not put together is 1- 1/5=4/5.