Where is the lost dollar? Somebody explained me this problem, but I am not sure to understand what is wrong.

Three people go to a bar to have a drink.
To pay the bill everyone gives $10.
The barman gives back $5 coins.
Every people take $1 back and leave the $2 other dollar for the barman.


If everybody give \$10 and get \$1 back, they paid \$9 right?

3 * $9 + $2 = $29


Where is the missing dollar?
True $3*9=27$.

But they paid $25\$$ and gave $2 \$ $ tip. And 

$$25+2=27=3*9 \,.$$
The math is simples:


10*3 = 30 and 30-25 = 5
(1*3)+2= 5


∴

There is no loss.