Each year, students in an elementary school take a standardized math test at the end of the school year. for a class of fourth-graders, the average score was 55.1 with a standard deviation of 12.3. in the third grade, these same students had an average score of 61.7 with a standard deviation of 14.0.

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.