Currently, you have a 90, 78, 88, and an 80 on your

tests in math. What do you need to make on the next test

in order to get an average of an 85?.

After next test you will have passed 5 tests. In order to get 85 on avarage you should have total 85*5 scores = 425. Subtract already gained scores'

425 - (90+78+88+80) = 89. So You should get 89 on your next test to achieve 85 score avarage.