Find the positive value of x that makes the following equation a true number sentence. x^{2}=81^{-1}

Find the sum of 4x^{2} -4+6 and -7^{2} +3x+1

If 10 men take 18 days to mow 60 acres of grass, how long will 15 men take to mow 80 acres?

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.