3 men go into the restaurant at the Hotel Hilbert, where they agree between them to pay exactly a third of the bill and the waitresses tip, whatever the individual costs. At the end of the meal, the waitress brings them the bill, which comes to �25. As this amount is impossible to divide precisely into 3 equal parts, they each put �10 on the saucer. The waitress brings them �5 change, and each man takes �1, leaving a tip of �2 for the waitress. This means that each man has just �1 left from his initial outlay of �10 - which means that each man paid out �9. 3 x �9 = �27, and the �2 left for the waitress makes it �29. So what happened to the other �1?
This chestnut rears its head in different stories every so often. The maths doesn't add up only because of the wrong wording in the question. The �27 spent includes the tip, why should it be added on again?