10 books can be rearranged in 10! ways consider the two books taken as a pair then number of favourable ways of getting these two books together is 9! 2!
Required probability = 1/5
10 books are placed at random in a shelf. The probability that a pair of books will always be together is
* 1/10
* 9/10
* 1/5
* 3/10
* 1/2