Rose likes to water ski using her sister's boat. The boat takes 3 gallons of gas each hour to run. Gas costs $5 per gallon, how much money will she spend on gas to ski for 4 hours?

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.