ANSWER:
The facts in this riddle are clear: There is an initial $30 charge. It should have
been $25, so $5 must be returned and accounted for. $3 is given to the 3
friends, $2 is kept by the bellhop - there you have the $5. The trick to this
riddle is that the addition and subtraction are done at the wrong times to
misdirect your thinking - and quite successfully for most. Each of the 3 friends
did indeed pay $9, not $10, and as far as the friends are concerned, they
paid $27 for the night. But we know that the clerk will tell us that they were
charged only $25 and when you add the $3 returned with the $2 kept by the
bellhop, you come up with $30.