Which equation is correct regarding the measure of MNP?

m MNP = 1/2(x-y)
m MNP = 1/2(x+y)
m MNP = 1/2(z+y
m MNP = 1/2(z-y)

Astudent must have an average on five test that is greater than it equal to 80% but less than 90% to receive a final grade of b. devon's greades on the first four test were 78% 62% 91% and 80% what range if grades on the fifth test would give him a b in the course? ( assuming the highest grade is 100%)