mathematically, what happen to it? say if they where to just pay 9$ each to begin with
Nothing happened to it. You were just tricked by the question to count the wrong number (3 instead of 2 or vice versa if you thought the other way around)
Total 30 was paid. Of the money moving around...
25 dollars was owed. that leaves the extra 5 dollars. +2 for the butler, +3 to the 3 guys = 30
Now, what you did was this. 30-3+2=29 which is incorrect. Total 30 dollars was paid, which means 27. The remaining 2 dollars isn't added back up to the total, it's supposed to be subtracted downwards to 25 to display the total that was actually owed. (You end up moving the 2 dollars in the wrong direction)
Idk... it makes more sense if you look at it from an accounting perspective... but since I bet no one loves accounting...
Sardine lolipop.