We make every assignment equal to 100%, with each question worth an equal percentage of the total.
If there are 4 questions, each is worth 25%. If there are 5 questions, each is worth 20%.
Short answer questions are worth the same percentage as multiple choice questions, but they can have partial scores; so if there are 4 questions and 1 is short answer, that short answer is worth 25%, and a 2 out of 4 on that short answer would equal 12.5% out of a possible 25 percentage points.
Finally, we only count Part B questions when the students get Part A correct. We discount Part B results from the total score when students get Part A wrong. We drop Part B from the total regardless of whether they get it right or wrong. So if there are 4 questions and #1 is Part A and #2 is Part B, and the student gets Part A right and Part B wrong (and number 3 and 4 correct), then the student gets a 75%. But if the student gets Part A wrong (and 3 and 4 correct) then the student can only get a 67% because we only count questions 1, 3, and 4 towards their total score.