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.

From Ward, another way to look at this one: My answer:

The following night two friends check into the same motel. Once again the
clerk charges them $30, or $15 per person. After the clerk remembers the
total rate is only $25 he sends the bellhop upstairs with five $1 bills to pay
the two friends back. The bellhop knows he got away with larceny once
so he tries it again. But this time he pockets $3 and returns $2 to the hotel guests ($1 per
guest). So each of the two guests got $1 back from their
original $15. Therefore each paid $14 which is a total payment of $28
for the room. Now the bellhop has $3, the guests paid $28, for a total of $31....THERE'S
THE MISSING DOLLAR!!!!