missing dollar problem
Three people have dined at a restaurant and received a total bill for 30ドル. They agree to split the amount equally and pay 10ドル each. The waiter hands the bill and the 30ドル to the manager, who realizes there's been a mistake and the correct charge should be only 25ドル. He gives the waiter five 1ドル bills to return to the customers, with the restaurant's apologies. However, the waiter is dishonest. He pockets 2,ドル and gives back only 3ドル to the customers. So, each of the three customers has paid 9ドル and the waiter has stolen 2ドル making a total of 29ドル. But the original bill was for 30ドル. Where has the missing dollar gone? (See solution below.)
A version of this problem first appeared in R. M. Abraham's Diversions and Pastimes in 1933.1 See also nine rooms paradox.
Solution
There is no missing dollar (of course!). Adding 27ドル and 2ドル (to get 29ドル) is a bogus operation. They paid 27,ドル 2ドル went to the dishonest waiter, and 25ドル went to the restaurant. You have to subtract 27ドル minus 2ドル to get 25ドル. There never was a 29ドル; it's a phony calculation designed to confuse the unwary.
Reference
1. Abraham, R. M. Diversions and Pastimes. London: Constable & Co., 1933. Reprinted, 1964.