The half-life of cesium-137 is 30 years. Suppose we have a 150-mg sample. (a) Find the mass that remains after t years. y(t) = mg (b) How much of the sample remains after 60 years? (Round your answer to two decimal places.) mg (c) After how long will only 1 mg remain? (Round your answer to one decimal place.) t = yr

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.