Puzzle (from the book - ways of thinking)

Three travelers go into a hostel. Each of them pays 10dollars for the accommodation. Later, however, the innkeeper remembers that three persons in a room should pay only 25dollars, so he sends 5 dollars back to them by servant. The servant, however, thinks that this is an unexpected stroke of luck for the travelers anyhow, and it is enough to give only 1 dollar to each of them. So he can slip 2 dollars into his own pocket. Now, each of guests ends up having paid 9 dollars and the servant put 2 into pocket. This amounts to 3X9+2=29 dollars. Where is the thirtieth dollar?