The formula for average is:
For the 8 tests, the average is presented as 91. While we don't know the exact
score on each test, you have to use the information that is given. Since the average
of all previous scores is 91, assign the score of 91 to each of the 8 previous tests.
This gives us 8 tests with a score of 91 each.
91 \u00D7 8 = 728
To get new average after the 9th test, use the same formula:
Since there are 2 answer choices that are very close to this, and you don't know which way the .5 will be rounded, extend the division another place by adding a 0 to the dividend, then divide.
You find that the answer is now 89.55, which rounds up to 89.6. Therefore, you now know that 89.5 would not
be correct. Always extend the decimal places when 2 answer choices are as close as these and you don't know which one is correct.