Please help quick. I will mark brainliest. ik the answer is 19 because a is 18 and b is 1, but someone please solve it algebraically for me

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%)