Question 285148
A different way to solve it is which shows why is:

To get an average of 90 we find out how much he other score was above or below 90

90 even
78 -12
82 -8
 total -20
so on the next test the student must get 90+20=110
not very likely  
The best the student can hope for of course is 100 which would give an average of 87.5.