Answer:
11. This is counting only odd numbers, or counting by two starting with one.
X=3/2 or 1/2 I think that
You should be able to multiply 91 by 8 to get the sum of the first 8 tests, then add the score on the 9th test. Then to get the average of the sum of the 9 tests you need to divide it by 9.
((91*8)+78)/9=89.555555555555.... So, 90%