I don't understand the third method to calculate degree classification

So, my university works out the degree classification using three different methods. The first two make perfect sense to me. It is either weighted 40:60 between 2nd and 3rd year, or based on purely on 3rd year results (after discounting the lowest 20 credits).

However, this third one I really don't understand what equation this one is trying to communicate.

c) the minimum classification in which more than 50% of the combined relevant credits at Level 5 and Level 6 were attained after first discounting the marks in the worst 20 credits both at Level 5 and at Level 6.