I am trying to apply zero marking in a multiple choice exam. I want students to get max_grade for one determined combination of answers and get 0 in other case. Ex: I have some question and 5 answers. The combination of 1,2 and 4-th answers is correct. Any another combination is wrong. Setting 20% to right answer and -20% - to wrong gives 0.2*max_grade instead of zero in case student's answer is (for example) 1.
Will be grateful for help.