So if we follow the spirit of the puzzle, why should you have paid for all the beer?

>We don't need to assume anything. We have 8 bottles, so each of us drank 8/3.
>John gave 5 and drank 8/3, so 7/3 is the difference
>Bill gave 3 and drank 8/3, so 1/3 is the difference.
>So, they should divide the money in proportion 7:1
>
>
>>If we assume 1$ a beer. If it is !.50 a beer, then you didn't pay enough. Hence the x in my answer.
>>
>>>>>John brought 5 bottles of beer, Bill brought 3 bottles, when I came. So, we drank this beer together and then I gave them $8 (all I had at this moment). How should they divide this sum?
>>>>
>>>>$7 for John, / $1 for Bill.
>>>
>>>And * for you :)