There were 43 children at the playground. Now there are 27 children at the playground. How many children left the playground? Ben says that 13 children left the playground. He thinks that 43 – 30 is easier to subtract than 43 – 27. He subtracts 43 – 30 and gets 13. Why is Ben's reasoning not correct?

What situation is an example of an observational study

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.

If 7x=9y, y/z = 7/5 , then x/z = (a) 2/7 (b) 7/9 (c) 9/5 (d) 9/7 (e) 5/9