If 76 was your only previous grade, then your new average would be
(76 + 80 + 100) /3 = 85.3
However, if 76 represents an average of some other grades, then, this would be a hard one to answer.
Consider that your grades could have been 90, 76, and 62. This would be an average of 76.
When we add 80 and 100, the average is now (90 + 76 + 62 + 80 +100)/5 = 408/5 = 81.6
However, suppose your grades were 78 and 74. This is also an average of 76. Now add 80 and 100, and we get (78 + 74 + 80 + 100)/4 = 332/4 = 83
So, in short, we would need more information about your previous grades to give a definitive answer in the second situation.
