Mae sold raffle tickets to steven and to theo. each ticket costs k dollars. steven bought 10 raffle tickets. theo bought 15 raffle tickets. which expression must be equal to the difference between how many dollars theo spent and how many dollars steven spent

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.