Sophie is building a wheelchair ramp onto her porch. If the porch is 48 centimeters high and Sophie wants the ramp to start 64 centimeters from the porch, how long does the ramp need to be?

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.