Calcium-51 has a half-life of 4 point 5 days. Only 0 point 7 5 gram remains of a sample that was initially 12 grams. How old is the sample of calcium-51?
Answer
A
4 point 5 days
B
18 days
C
13 point 5 days
D
9 point 0 days
Explanation
Each half-life reduces the sample by half: 12 g → 6 g → 3 g → 1.5 g → 0.75 g. That is four half-lives, and four times 4.5 days is 18 days. So the sample is 18 days old.
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