Situation: A 22 gram sample of a substance that's used to sterilize surgical instruments has a k-value of 0.138. N = Noe -kt No = initial mass (at time t = 0) N = mass at time t k = - a positive constant that depends on the substance itself and on the units used to measure time t = time, in days

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.