Remember number of goals can only take a whole number - if you've got just 1 goal scored in the second half then how can you have 13/20 scored by the teams outside the top 6? Or 7/20 scored by teams in the top 6?
You need multiples of 20. Likewise for the first half you need multiples of 5.
so 5 goals in the first half and 20 in the second is valid, 10 goals in the first half and 20 in the second is too and so on...

