It means the three men paid $27($9 each for a room which cost $25,meaning their change is the $2 left with the bell boy i.e $30 left their pocket instead of $25,they had a change of $5 they where giving $1 each by the bellboy who kept $2,no money was missing-you should have subtracted the $2 with the bellboy from the $27 paid by the 3 men to equate the $25 now in the account of the man at the desk not adding it