Kind of reminds me of this oldy but goody...
Three guests check into a hotel room. The clerk says the bill is $30, so each pays $10. Later the clerk realizes the bill should only be $25. To rectify this, he gives the bellhop $5 to return to the guests. On the way to the room, the bellhop realizes that he cannot divide the money evenly. As they didn’t know the total of the revised bill, he decides to just give each guest $1 and keep $2 for himself.
Now that the guests have been given $1 back, each has paid $9. Notice that 3 x $9 = $27, and the bellhop has $2. But 2 + 27 = 29. If the guests originally handed over $30, what happened to the remaining $1?