If the half-life of k-40 is 1.3 billion years and if a sample rock found on an island is estimated to be 1 billion years old, what percentage of the original parent k-40 would be present in the rock? round your numerical values to the hundredths decimal place.

answer choices

(a) 76.92%
(b) 1.3%
(c) 58.67%
(d) 99.99%

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.