[n" different books "(n>=3)" are put a random in a shelf.Among "],[" these books there is a particular book "A" and a particular "],[" book "B" .The probability that there are exactly "r" books "],[" between "A" and "B" is: "]

n different books (n>=3) are put at random in a shelf.Among these books there is a particular book A and a particular book B.The probability that there are exactly r books between A and B is (A) (2)/(n(n-1)) (B) (2(n-r-1))/(n(n-1) (C) (2(n-r-2))/(n(n-1)) (D) (n-r)/(n(n-1)

In how many ways can 7 different books be arranged on a shelf? In many ways three particular books are always together?

Number of ways in which 15 different books can be arraged on a shelf so that two particular books shall not be together is

Number of ways in which 15 different books can be arraged on a shelf so that two particular books shall not be together is

How many different selections of 4 books can be made from 10 different books,if There is no restriction Tow particular books are always selected; Two particular books are never selected?

How many different selections of 6 books can be made from 11 different book , if Two particular books are always selected ,

How many different selections of 6 books can be made from 11 different book , if Two particular books are never selected ?