Question 286457
Let s1=score 1, s2=score 2, s3=score 3, and x=the unknown score

The formula for average is:

Avg = (s1+s2+s3+x)/4
90= (88+79+94+x)/4

360=261+x
x=99

He would have to get 99 on his 4th exam in order to get 90 average.