Marissa has taken four 100- point tests in math this semester. her mean score is 89%. what score does she need on test 5 so the mean will be 90%

I mean, the mean could be 4 89s, so having a 94 on the test will boost her grade to a 90, as the mean is now higher with the fifth value.

Her total score for the 4 tests is 4*89 =   356

so to get a mean of 90%  she has to get  5*90 = 450 marks total
So in test 5 she needs to get 450 - 356 =  94 marks