Question 872590
Let x = score on the fifth test


The scores for the five tests are: 83, 86, 91, 77, x


They add to  83+86+91+77+x = 337+x


Divide that sum by 5 to get the average. We want this average to be equal to 85. Set it equal to 85 and solve for x.


(337+x)/5 = 85
337+x = 85*5
337+x = 425
x = 425-337
x = 88


He needs to get an <font color="red">88</font> on the last test.