Identify the property being demonstrated for each expression. 13+ (5 + 7) = (3 + 5) + 7
(3)[5(7)] = (3)[7(5)
5 + 0 = 5

There are 45 questions on your math exam. you answered 810 of them correctly. how many questions did you answer correctly?

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.

What is the ratio fraction form of 25: 9