It seems to me that you're only giving us a grain of sand when the problem involves the entire beach.
Where you say "If I have an exam which is worth 50% pass mark" are you saying that your exam constitutes 50% of the final grade? Or are you saying that you need 50% on the exam to pass?
In either case, you need to tell us how many questions are on your exam. You also need to tell us the percentage of marks you already have towards this subject.