3. A student borrows £3500 to be repaid after 3 years. The lender quoted a flat rate of interest of 6.25%. Calculate the total interest that should be paid.

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.