Eyefusion:

Actually your conclusion is wrong on the following.

"After they got back 3 dollars, their cost was 9 dollars a drink"
"As we can see, their cost was not $9, Their cost should have been $8.3333 per person."

I understand Ratobhale wanted to confuse us. By saying "cost is $27 plus 2 = 29 where does $1 go?" But the thing is cost is $27 [25 (taken by seven eleven)+ 2 (assistant)] right. He should have added $3 that they got back from assistant, NOT $2 which is a part of cost.

Krishna and GP are getting the gist of it. They are correct.