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
Options:
[A] 1/9
[B] 1/7
[C] 1/5
[D] 1/6
[E] 1/3