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) 1/7
2) 1/9
3) 1/5
4) 1/8
5) 1/2