Jamie is making bracelets using black and red beads. she uses 6 black ones for every 4 red beads. how do i represent the number of black beads for any given number of red beads using an equation and how do i identify the constant of proportionality?

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%)