Two trains leave the train station at 10:00 a. m. and travel in opposite directions. Their rates are 65 mph and 85 mph. How many hours will it take for them to be 450 miles apart?

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.