There are three exams in a marking period. a student received grades of 75 and 81 on the first two exams. what grade must the student earn on the last exam to get an average of no less than 80for the marking period

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