Question 693129
The average grade is the sum of grades divided by the number of grades. Your student has an 84 (out of 100) average for 5 grades, so the sum of the first five grades is
(1) sum = 5*84 or
(2) sum = 420
Now when the student scores 100 on each of the next three tests his new sum is 
(3) new sum = 420 + 100 + 100 + 100 or
(4) new sum = 720
Now he has 8 grades, so his/her new average is
(5) new average = 720/8 or
(6) new average = 90
Answer: The student's new average will be 90%.