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 20 points.

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

For example, if only 3 questions were asked in class and you got one wrong, then your quiz grade for that day is 66.7%. One forum contribution would get you up to 86.7% and it would take two forum contributions to completely repair the damage.