Let x be the solution at 20% and y be the solution at 40%, hence
x + y = 300 ml
We now write an equation that expresses that the total acid in the final 300 ml is equal to the sum of the amounts of acid in x and y
28% * 300 = 20% * x + 40% * y
Solve the above system of equations to find
x = 180 and y = 120