Question 969361
Average grade of 90 for 3 exams means his total score must be 270, for (270/3)=90

He got a 96 and an 80.  They add to 176

270 (what he needs)- 176=94.

If he gets a 94, a 96, and a 80, he gets 270.  

Another way is he was +6 on the first exam and -10 on the second.  He is down 4.  He needs to be +4 on the third, and that is a 94.