Each task has a % weighting of final overall mark
Those % are added up to form your overall mark
Whoever has the highest % in the overall mark is ranked first, 2nd highest % ranks second and so on.
For example, if a task weighs 25% and you get 8/10 on it, you got 80% on that particular task, this means you will get 80% of the 25% added to your overall %.