Mr. sheng is using a faucet attachment to fill the fish tank in his biology classroom. the graph below shows the amount of water in the tank during the first several minutes since he turned on the faucet what is the slope of a line joining the points on the graph? if your answer is not an integer, express it as a decimal

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.