What is the solution to the system of equations? {x+3y-z=2 4x+2y+5z=1 3x+z=12 answer is (5,-2,-3)

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%)