Question 722812
{{{x}}}= grade for the third test.
For the average after the third test to be 80, the sum of all 3 test grades divided by 3 must be 80.
{{{average=(76+81+x)/3=80}}}
Multiplying both sides of the equal sign times 3, we get
{{{76+81+x=80*3}}} --> {{{157+x=240}}} --> {{{x=240-57}}} --> {{{highlight(x=83)}}}
That is probably the expected answer.
 
However, in real life, thanks to rounding, Susan could get away with an 82, because
{{{76+81+82)/3=235/3=79&2/3}}} and that rounds up to 80.