What's the property that justifies my 1st step?

How to solve a simultaneous equation involved with fractions?

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%)

Me d is also an option but i couldn't get it in the picture