Its simple trickery , there is no missing pound.
Dead easy to solve with my 130iq.
Everyone is stuck on the idea that because they gave £30 and got a pound each back they have therefore paid £27 ie £9 each.
Indeed this is absolutely correct. The problem comes you forget to add what was stolen by the employee into the bill.
They paid £25 for the rooms and £2 unknown 'tax' to the employee (Total £27)
They paid with £30 and got £3 change.
There is no missing pound
simples.