A relative score calculation

Hi,
I want to create a learning unit in Storyline that will report a relative score based on the number of questions the learner chooses to answer.

If they answer 10 questions, they will receive a score as if those 10 questions represent 100%, and if they answer 7 questions, they will receive a score as if those 7 questions represent 100%.

The questions the learner chooses are random, and I cannot predict which ones they will answer. Is there a way to do this?

Thanks a lot