There is a five –volume dictionary among 50 books arranged on a shelf in a random order. If the volumes are not necessarily kept side-by-side, the probability that they occur in increasing order from left to right is:

None of these

Probability = $\frac{(Selection\, of\, 5\, places\, for\, dictionary)×(arrangement\, of\, 45\, books)}{Total\, arrangements}=\frac{{^{50}C}_5×45!}{50!}=\frac{1}{5!}$