THE’Riddle of the three friends At the restaurant it is based on a very simple account, but in its simplicity it makes everyone crazy, because it seems to appear a missing euro. The situation is this: three friends at the restaurant see a account of € 25and having only banknotes in their pockets 10 € per person to pay the bill, for a total of 30 €. At this point, the waiter brings them € 5 of the rest, who however do not know how to divide himself being three! Thus they decide to keep € 3 taking a euro per person, lowering the expense of each of them to € 9 instead of € 10, and leaving € 2 of the waiter.
And here comes the surprise: they spent € 9 each, so € 27 being 3, which added to the 2 € of Mancia lead to a Total of … 29 €! Yet the euros of departure were € 30! But then, where does the missing euro end?
For the most attentive, the solution may seem obvious. This riddle is in fact based on adeceit obtained from shifting attention to the wrong figure, that is 30 €, and on the fact that we have counted the money several times of due. Let’s try to understand it better. The amazement of those who are deceived derives from the fact that number 29 and number 30 are very close to each other and seem to create the mystery of the disappeared euro, but attention should not be paid to the number 30, but on the 27th, that is, the real amount of the spending of the three friends.
When the 5 euros arrived, they were divided into two parts: € 3 who return to the hands of friends and € 2 who remain as a tip. Let’s focus on € 3: these must be subtracted from € 30 of initial expenditure, because they have been returned, bringing the expenditure from 30 € to € 27. On the other hand, the account was € 25, to which the € 2 must be added to the tip … which already fall within the expense of the three friends! At this point, the accounts return: the three friends have spent 27 €, that is € 25 of account + 2 € of Mancia.
