Bunny, Flower and Duka decided to have a stay at Dynasty Hotel.   The hotel rate was S$30 per night.  Each of them came out S$10 and paid at the counter upon check in.   Later the hotel staff realised that there was an error in the system.  The hotel rate for the night is S$25.  He gave S$5 to the A-bo, the bell boy.  He told him A-bo to return the money to the three guests. 

In the lift, A-bo stared at S$5 and wondered wondered how should he divide the reminding change to these people.  As he pressed the door bell, an idea struck him.  He returned $3 to them, each of them got back $1 each.  A-bo kept the $2 for himself. 

I need your help to with this problem sum..
If each person paid S$9 for the room, total sum is S$27.  If add up A-bo's S$2, that arrive S$29, so where does the one dollar go to?