The final marking formula cannot currently be changed.
Perhaps you can set the questions scoring strategy to get your result, giving appropriate values to m (score for a wrong answer) and b (score for a right answer).
You need a final score equal to 5.5 for 15 right and 5 wrong answers : 15 b + 5 m = 5.5
You need a final score equal to 10 for all right answers : 20 b = 10
So you can try

\scoringDefaultS{b=0.5,m=-0.4}

(I'm sorry I did not try it, perhaps you'll have to tweak it).
Note that with this scoring strategy, giving 15 right answers and not responding to others will result to a final score equal to 7.5.

I'm assuming you've though of using an "off-line" strategy (Export the file to OpenOffice (you have to change the export option in the Reports tab), to get a spreadsheet and can add a column with a formula to adjust the grade of each student's exam.)

Of course, that doesn't change how the exams are marked, but does do a quick calculation for you.

Cheers!
Devin