The sample decreased to [tex]\frac{1}{8}[/tex] of its original size, so it underwent [tex]\log_{2} 8=3[/tex] half-lives.
One half life is then
[tex]\frac{36\ min}{3}=12\ min[/tex]
One-half of 24 grams is 12 grams, so 12 grams remained after the first 12 minutes.
