Cory's average score after 8 tests is 91%. If he gets a 78% on his 9th test what will his new average be?

add all the scores together and the divided by the number of scores there are

8 tests will be 8x91 = 728

Then add on his latest test result 728+78 = 806

Now divide by the number of tests (9) 806/9 = 89.55555555

89.555%