Well, it depends on what the weight of the test is. Most courses tell you that a test is worth a certain percentage of your grade(i.e. Final Exam is worth 20% of your entire grade). It also depends on what your grades for other assignments were, and how many there were. Try asking your teacher.
The average of your previous grade and the grade you got on your test is 84.5%. Average is the mean, which is calculated by: (91 + 78)/2.
