Several years ago, I heard the following puzzle which is one of my all-time favorites. There are many variations. One version goes like this:

Three men on a business trip eat lunch at a restaurant. When they finished they got a bill for $30 so each man paid $10 to the waitress. When the waitress brought the money to the manager-cashier, the manager-cashier realized the bill should only have been $25. So the manager-cashier gives the waitress $5 to return to the three men.

The waitress realizes she can’t split the $5 evenly among the three men, so she decides to give each man $1 apiece and keep the extra $2 as a tip for herself, which she does.

OK, at this point, each of the three men got $1 back so each man paid $9, and $9 times three is $27. The waitress has $2. That’s a total of $27 + $2 = $29. The three men originally gave the waitress $30 so where did the missing $1 go?

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It’s a very clever puzzle that is based more on language than on mathematics. The trick is that the last part of the puzzle where $9 * 3 = $27 + $2 = $29 doesn’t really account for all of the money because the manager-cashier has $25. If you actually take $30 (in real money or just pieces of paper) and distribute the money according to the riddle, you’ll see that there really isn’t a missing dollar.

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It’s common to call this problem “the missing dollar riddle” but it’s really more of a puzzle go me. A riddle is usually a play on words, for example, “It goes in the morning on four feet, at lunch-time on two, at evening on three.” The answer is “man” — crawling as a baby, walking as an adult, using a cane as an old man.

Left: “Double your IQ or no money back!” “Okay, sounds good to me.” Right: Exiting a Mind Reader — “Let me guess, she gave you your money back.”