I created a quiz that had a "do three of the following five" format. When I mark student's answers out of ten the overall grade is calculated out of 50 (because there were five questions) instead of 30 (they were required to do three). For example a student may score 9/10 on all three questions which is 27/30, however this is reported to the student as 16.2/30. This is just a little disconcerting to students!
(It is easy to see how Moodle ends up with this calculation: 27/50 is equivalent in value to 16.2/30.)
I am not sure if this question fits best here or in the Gradebook forum....
Moodle does not support this sort of quiz at the moment. You will just have to work around it. Maybe make the quiz out of 50, so the score is reported as 27/50, which is slightly less confusing.