What is the equation for the problem, a store buys an item for $10 to earn a profit of $25 is what percent does the store need to markup the item

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%)