Several people have already answered this, but a lot of people don't seem to understand it so I'll give it a try.
The men didn't pay 27 dollars for the room, they paid 25. The desk clerk screwed them each out of 66.66 cents, which he pocketed. The desk clerk TOLD them that they were paying 9 dollars each, in reality, they were paying $8.33 each ($25) and he stole the other 66 cents each (2 dollars) that they thought they were paying for the room. It's like this.
$30 what they paid.
- $3 what they got back.
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$27 what they thought they were paying for the room
-$2 what the desk clerk stole from them
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$25 the cost of the room
It's worded to confuse you, because you shouldn't be adding the 2 dollars that the clerk stole to the 27. The 27 includes that 2 dollars already (25 for the room and 2 for the clerk = 27 what they thought they were paying). Then you add the 3 to that to get the original thirty.