It depends on how many questions were asked in class that day. A quiz, which consists of all of the questions asked in a given class, is worth 100 points while a forum contribution is worth 7 points.

For example, if just 5 questions were asked in class and you got one wrong, then your quiz grade is 80% and you would need three forum posts to make up for the question that you missed.

For example, if 15 questions were asked in class and you got one wrong, then your quiz grade for that day is 93.3%. One forum contribution would completely repair the damage.