Nine Bottles
There are nine bottles, each containing a different type of liquid,
but the labels have all fallen off. Knowing nothing about the contents, a
passerby reapplies the labels at random. What is the expected number
of correctly labeled bottles? (That is, if the labels are randomly
assigned many times, what is the average number of correct labels?)
It might be easier to try the problem for smaller numbers of bottles first.
Source: Internet newsgroup rec.puzzles, Used as Ken's POTD 5/16/94.
Solution
Mail to Ken