Your test scores in one class are 81 and 89. What possible scores can you earn on your next test to have a test average between 85 and 90, inclusive?
If the next score is x, then
3*85 <= 81+89+x <= 3*90
255 < 170+x <= 270
85 <= x <= 100
3*85 <= 81+89+x <= 3*90
255 < 170+x <= 270
85 <= x <= 100