I'm not confused - I've given an answer back here:
https://forums.overclockers.co.uk/posts/32318409/
for some reason Inogen has quoted me a bunch of times to argue that I shouldn't simplify the fraction I've given as my answer
I assume this.
Assuming 3/5 is the portion of the goals scored in the first half by the top 6 teams then we can't answer as we need to know what portion of goals were scored in the first half relative to the second half. Answers of 19/40 are nonsense.
If we define x to be the portion of goals scored in the first half relative to the second half then our answer is:
x(3/5) + (1-x)(1-13/20)
If x = 1/2 then that does indeed work out to be 19/40 but there is no reason to assume that it is.
Assuming there were 5 goals scored in the first half and 20 in the second half then of course we can answer: (3 + 7)/(5 + 20) = 2/5
You appear to have realised you can't just use both initial figures and they have no relation to each other. But then you try to force one in with the use of proportion, there is no proportional relationship between the first half and the second half. The 2nd half's number of goals is completely unrelated to the first half.
Lets take your final example of 5 goals in total in the first half and 20 goals in total in the second half.
So 25 goals in total.
How many goals did the top 6 score? 3/5 of 5 = 3. + 13/20 = 13. So a total of 16 goals.
So 16/25. and this can't be rounded down so the final answer is
16/25.
But as we don't know know how many goals were scored in each half which this is why I had to answer all the probable out comes.