On his first three tests, hiram got scores of 89, 95, and 90. his final test score brought his median score to 91.5. what was i final test score?

The median is the middle score, it is also the average of the two middle scores when total number is even.

original 3 scores:  89, 90, 95  > median = 90

with final score:  89,90,X,95  > median = 91.5

(90+x)/2 = 91.5
90+x = 183
x = 93

Final score was a 93.