Lets suppose that we have 4 math books, 6 statistics books, and 10 books of other subjects, and we want to order a shelf with those books. The restriction we have is that the math and the statistic ones must be together in the shelf (I mean, the math ones should be together, and next to the statistics books). So the question is:
What is the probability about this event happens?
Remark: Every book is different