Average score on first 8 tests is 92, what is the minimum score needed on the ninth test to get an average of 90 in all the tests

Average=(sum of all scores)/(number of scores), in this case we see that:

92=s/8 so sum of the first eight test scores is:

s=8(92)=736 now we wish to know the minimum score on the next test that will result in an average of 90...

(736+s)/(8+1)90 multiply both sides by 9

736+s810 subtract 736 from both sides

s74

Thus a score of 74 or more is necessary to maintain an average of at least 90