81 is 45% of what number

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%)

Cd is the perpendicular bisector of both xy and st, and cy=20. find xy.

The weight of a full steel bead tire is approximately 800 grams, while a fighter wheel weighs only 700 grams. what is the weight of each tire in pounds?