Non linear score

Hi heroes, I have an E-learning with 20 questions (selected at random from 60 questions).

So if a learner answers 4 questions wrong he automatically will score 80%.

But....I would like that he scores 60% when 4 answers wrong.

AFTER 4 answers wrong every wrong answer should mean 5% lower total score.

Does anybody know how to achieve this?

Kind regards, Nicole