Hi,
first four tests(80% of grade) were 76%,65%,90%, and 80% Average = 77.75
student must have an average(the mean) on five tests that is greater than
or equal to 80% but less than 90% to recieve a final grade of a B
.80(77.75) + .20x ≥ .80
.20x ≥ 80 - 62.8
x ≥ 17.2/.2
x ≥ 86 ***** B the highest grade possible
.20x < 90 - 62.8
x ≥ 27.2/.2
x ≥ 138%