A mixture of 13% disinfectant solution is to be made from 11% and 17 % disinfectant solutions. How much of each solution should be used if 18 gallons of the 13% solution are needed?

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.