Answer:
87
Explanation:
[] First we need to set up an equation
= 88
[] What does it mean? We want all of his scores plus the 5th test score (x) to equal an average of 88 so we must find the average of all of his test scores.
[] Now to solve!
First, let's multipy everything by 5 to remove the fraction.
91 + 94 + 78 + 90 + x = 440
Then, simplify
353 + x = 440
Now subtract the 353 from both sides
x = 87
Which results with our answer, he needs to get an 87!
Hope this helps, have a nice day :)