average means add all scores and divide by number of scores earned
last score is x
so the sum is 75+81+x
3 scores so (75+81+x)/3=average
must be no less than (greater than or equal to 80)
(75+81+x)/3≥80
solve
times both sides by 3 to clear fraction
75+81+x≥240
156+x≥240
minus 156 from both sides
x≥84
the studen must earn at least 84 on last exam
