Mandy has 50 yards of fabric to make costumes for a play. she makes 12 skirts that take 3 yards each, and 9 hats that each take 1 yard. which expression represents the number of yards of fabric that are left?

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.