Calcium-51 has a half-life of 4.5 days. Only 0.75 gram remains of a sample that was initially 12 grams.How old is the sample of calcium-51?

Each half-life cuts the sample in half: 12 g becomes 6 g, 3 g, 1.5 g, then 0.75 g. That is four half-lives. At 4.5 days per half-life, the sample is 4 × 4.5 = 18 days old, so the correct choice is 18 days.

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