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Somebody explained me this problem, but I am not sure to understand what is wrong.
Three people go to a bar to have a drink. To pay the bill everyone gives $10. The barman gives back $5 coins. Every people take $1 back and leave the $2 other dollar for the barman.
If everybody give \$10 and get \$1 back, they paid \$9 right?
3 * $9 + $2 = $29
Where is the missing dollar?