You've probably seen this trick question floating around the Internet before:
If 1 = 5, 2 = 10, 3 = 15, and 4 = 20, then 5 = ?
The question is twofold:
What is the trick answer they expect? (You can use Google to answer this one.)
What is the most elegant way to define the equivalence relationship between numbers using the logic of the trick answer? (i.e. suppose that the answer you got in part 1 was indeed the unique answer to the question, and all the numbers on the right side of the equation were the unique answers to their "equalities", how would you explain the pattern for other numbers? In particular, what would 25 equal?)