The university introduced a new advanced music class a few years back. The below table gives the number of students who participated in the advanced music program for the first 4 years. Find the value of the slope and interpret it in words. Years (x) 1 , 2, 3, 4 Students (y) 20, 40, 60, 80

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.