This whole tutorial starts with a simple example problem Suppose we have 99 empty papers and we wrote numbers from 1 to 99 (using each number) on one side of the papers randomly. Then we mixed all the papers randomly and started to write numbers from 1 to 99 on the other sides (empty sides) of each paper randomly. What is the probability of having the same number on both sides of at least one paper? https://math.stackexchange.com/questions/394007/a-tricky-probability-question Give it a go and quickly you will see how tricky it is. The answer requires an understanding of derangements and we will go through this idea in detail in this post. What is a derangement? So what is a derangement? Consider a[…]