The number of customers in a grocery store is modeled by the function y = -x² + 10x + 50, where y is the number of customers in the store and x is the number of hours after 7:00 a. m. a. At what time is the maximum number of customers in the store?

b. How many customers are in the store at the time in part (a)?

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.