The ratio of boys to girls at a movie is 4:7. If there are 21 girls, how many boys are at the movie?

Measures of the angle of r, 31 s (x+4) t (3x+9)

Me. i can't figure out this question. give an explanation too. .

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.