A 12% brine solution was mixed with a 16% brine solution to produce a 15% brine solution. How much of the 12% solution and how much of the 16% solution were used to produce 40 L of the 15% solution?

Set $r$ is a set of rectangles such that (1) only the grid points shown here are used as vertices, (2) all sides are vertical or horizontal and (3) no two rectangles in the set are congruent. if $r$ contains the maximum possible number of rectangles given these conditions, what fraction of the rectangles in set $r$ are squares? express your answer as a common fraction.