the average score you achieve on the exams will not matter by itself for your final grade. only your score relative to your classmates matters. if everyone else gets 2 out of 50 points, for instance, while you get 3, your grade will be an A+. if everyone else gets 47 out of 50 points but you get only 45, your grade will be a D. of course, the distribution of raw scores is typically much wider so that final grades are well assigned by the curve. given voluntary withdrawals, the letter grade F is rare and assigned only if a student's performance falls below an absolute minimum, independent of the class-wide raw score distribution.
