According to the rate of water charge the minimum cost upto 27 units is Rs. 1910 and then Rs. 72 per unit for additional use. If a household used 30 units of water then find water charge to be paid.​

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.