In how many ways can 10 books be arranged on a shelf such that a particular pair of books should always be together?	Options
A) 9!  B) 2!  C) 3! * 2!  D) 9! * 2!  E) 9! * 5!
We have a total of 10 books.   Given that a particular pair of books should always be together. Hence, just tie these two books together and consider as a single book.   Hence we can take total number of books as 9. These 9 books can be arranged in 9P9 =9!=9! ways.   We had tied two books together. These books can be arranged among themselves in 2P2 =2!=2! ways.   Hence, required number of ways   =9!×2!   ANSWER 9! * 2! 9! * 2!
<result>D</result>
In how many ways can 10 books be arranged on a shelf such that a particular pair of books should always be together?