River Fun Boats rents paddle boats. The euqation C = 0.10t + 4 gives the charge in dollars, C, for renting a paddle boat for t minutes. What is the charge for renting the paddle boat for 20 minutes?

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.