Where's the missing dollar?

Each man had at fist paid ten dollars.When each got one dollar back from the assistant ,it means each man had actually paid only one dollars.There were three men . ＄9X3=27.The assiatant kept ＄2.＄27+＄2=29.Where's the missing dollar ?

3 × 9 = 27 减掉服务生藏起的2元＝25元 +每人的3元+服务生的2元=30元