The student officers are buying packs of streamers and balloons to decorate for school dance. Packs of balloons cost $3.50 and packs of streamers cost $2. If the student officers bought a total of 12 packs of decorations and spent $31.50, how many packs of balloons did they buy? How many packs of streamers did they buy?

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.