there are two ways to answer this....

The first answer is to think in terms of two seperate transaction:
The first is they pay £30 for the rooms.
Then the second is that they should be refunded £5, but are only given £3 of it. So of the £30 they initially paid out £25 is actually for the room, £3 is refunded to them and £2 is kept by the porter. and if you total that all up, it equals £30.

Or the second way of thinking about is, if they were all refunded a pound from their ten pound, the they did indeed pay £9 each. 3 x £9 = £27 and minus the £2 the porter kept = the £25 for the room.

I think that is correct....i need a sit down...!