Question 1208352
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Your equations would only be right if the final exam counted only as much as each test.  You need to weight the score on the final appropriately.<br>
There are 4 test scores which count for 1/3 of the grade, and the final counts or 2/3 of the grade.  Since the final counts twice as much as the 4 tests, the final has a weight equivalent to 8 tests (making 12 "tests").<br>
So to answer the two questions by the method you are using, you could use the equations<br>
(86+80+84+90+8A)/12 = 90<br>
and<br>
(86+80+84+90+8B)/12 = 80<br>
That first equation will give you the same answer as shown by the other tutor using a different method.  As he did, I leave it to you to solve the second problem to find the score needed to get a B.<br>