Mari and Jack each bought a sofa from a furniture store. Mari’s sofa cost $600. She paid $200 when she bought it and pays $20 per month. Jack’s sofa cost $810. He paid $140 when he bought it and pays $50 per month. After how many months will Mari and Jack each owe the same amount on their sofas? a)2
b)4
c)5
d)9

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.